Fix installation steps to allow for Pyenv 2

Fixes #394 - update pyenv virtualenvs to list virtualenvs start with dot prefixes

The text included info on Pyenv startup which is now outdated and incorrect.

Closes https://github.com/pyenv/pyenv/issues/1947, https://github.com/pyenv/pyenv-virtualenv/issues/387

~/.*rc should be modified instead of ~/.*profile

https://github.com/pyenv/pyenv-installer/pull/109

Otherwise, there's no effect for non-login interactive shells

Prefer `python3.x` executable if available

Add other way of deleting existing virtualenv

Improve fish installation instructions

Fix: support newer conda

also cleanup compatibility paths

pyenv-virtualenvs could not list conda environments & pyenv shell would only activate the base conda environment
the conda detection criteria of testing the presence of `conda` or `activate` files under `$(pyenv root)/versions/$version/bin` appears to be the culprit, since newer environments no longer include these files: those files reside in the base conda environment
- add detection criteria of `$(pyenv root)/versions/$version/conda-meta`
- compute the real prefix to the base environment from `realpath $(realpath $(pyenv root)/versions/$version)/../..`
- to allow that, enhance substitute `realpath` in `pyenv-virtualenvs` to reduce relative paths `.` & `..`, and factor that code out to a file under `libexec` for reuse
- hook `which` to locate conda from the real prefix

`psub` here is entirely unnecessary.

Update README.md

-Replace Mac OS X with macOS
-Added links

Updating ReadMe to include information for Fish

update README about PYENV_VIRTUALENV_VERBOSE_ACTIVATE

Use custom get-pip URL based on the target version (pyenv/pyenv#1127)

Refactoring around `GET_PIP_URL`

Manage `GET_PIP_URL` value at single place for readability.

Unset `PIP_VERSION` before invoking `get-pip.py` as a workaround for `invalid truth value` error

Source conda 4.4.4 shell files

Conda 4.4.4 (https://github.com/conda/conda/releases/tag/4.4.0)
made some changes on how to set up conda in the shell.
This commit addresses these.

This fixes some of the discussion in #178.

Move pyenv note to its own line

Add a note about virtualenv-init for fish users