References #730

The `../libexec` dance isn't necessary here. It was only necessary in
main `rbenv` command because that one might have been pointed to
directly via a symlink.

It doesn't try to chdir into RBENV_ROOT anymore because that might be
a location of an unrelated rbenv install that might have a different
version than the current one that is installed e.g. via a package
manager such as Homebrew.

Now just tries the repo where the source files (`libexec/*`) are
located, and if that isn't a valid rbenv repo, bail out early.

Closes #739

No need for helper function that's gonna be used just once.

It's only supposed to be set from `version-origin` hooks, but not
inherited from environment in case it was set.

Expose a `version-origin` hook.

It is invoked *before* the traditional `rbenv-version-file` lookup. Because `version-origin` is traditionally run immediately after `version-name`, then any plugin hooks that alter `version-name` would have done so. Thus, running `version-origin` prior to printing the origin gives those plugins a chance to alter the `version-origin` to match.

If any of the hooks set `$RBENV_VERSION_ORIGIN`, then it is used as the return value. Otherwise, the existing logic continues to return "environment variable" or "filename" as appropriate.

This change, in conjunction with the `version-name` hook, makes a clean seam by which plugins can inject their own ruby version setting logic. Using this seam, as opposed to altering `$RBENV_COMMAND_PATH` from the `which` hook, means that the version name and origin are set more reliably and so `version`, `version-name`, `version-origin` and `which` all work as expected. Indeed, even PS1 works now.

Expose a `version-name` hook.

It is invoked *after* the traditional `RBENV_VERSION` lookup. Which means hook scripts can interrogate `$RBENV_VERSION_FILE` and/or `$RBENV_VERSION` (or use the executables).

The hooks are then run, giving plugins a chance to alter `RBENV_VERSION`. Once the hooks have run, we now have (in `$RBENV_VERSION`) the actual version we want to use (or it's empty which defaults to `system` per normal). Lastly, the same logic remains for checking if the version exists, or trimming the `ruby-` prefix.

Prime example: the ruby-bundler-ruby-version plugin can select a ruby by using the `ruby` directive from the `Gemfile` if a local `.ruby-version` doesn't exist.

Fixes #807

Can be used for `.ruby-version` file lookup in the ancestry of a
specific directory. In this mode of operation, global version files
aren't taken into consideration, and the command fails unless a local
version file was found.

Add CPython 3.4.4

`OPENSSL_NO_SSL3` patch for 3.0.x, 3.1.x and 3.2.x

`OPENSSL_NO_SSL3` patch for 2.6, 2.7, 3.3 and 3.4

```
% dig python.org. A
;; ->>HEADER<<- opcode: QUERY, rcode: NOERROR, id: 9216
;; flags: qr rd ra ; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0
;; QUESTION SECTION:
;; python.org.  IN      A

;; ANSWER SECTION:
python.org.     5       IN      A       104.130.43.121

;; AUTHORITY SECTION:

;; ADDITIONAL SECTION:

;; Query time: 3 msec
;; SERVER: 192.168.44.2
;; WHEN: Wed Dec 16 06:08:20 2015
;; MSG SIZE  rcvd: 44
```

```
% dig www.python.org. A
;; ->>HEADER<<- opcode: QUERY, rcode: NOERROR, id: 22050
;; flags: qr rd ra ; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 0
;; QUESTION SECTION:
;; www.python.org.      IN      A

;; ANSWER SECTION:
www.python.org. 5       IN      CNAME   python.map.fastly.net.
python.map.fastly.net.  5       IN      A       103.245.222.223

;; AUTHORITY SECTION:

;; ADDITIONAL SECTION:

;; Query time: 7 msec
;; SERVER: 192.168.44.2
;; WHEN: Wed Dec 16 06:08:23 2015
;; MSG SIZE  rcvd: 83
```

Merge rbenv/master

Using `git merge rbenv/master -s recursive -X rename-threshold=5%`.

Conflicts:
	README.md
	bin/python-local-exec
	libexec/pyenv
	libexec/pyenv-help
	test/--version.bats

Confirms #836

Remove leading `:`