better Ubuntu Desktop installation instructions

Don't suggest that they add to their `.profile` anymore because:

1. They would have to restart the desktop session for their change to be
   reflected, or source the file manually;
2. An error in `.profile` may prevent logging in;
3. The `rbenv()` shell function and shell completions won't be available
   in Terminal since it doesn't start bash in login mode by default.

Therefore, suggest that they use `.bashrc` instead. This will be
immediately reflected in a new Terminal tab. If bash is started in login
mode somehow, the default `.profile` is set up to source `.bashrc` anyway.

Also, don't suggest restarting the shell environment with
`exec $SHELL -l`, since we don't know what was the original mode that
their shell was started in. (OS X Terminal.app will be login mode,
Ubuntu Terminal has non-login mode by default.) Mode can be checked with:

* bash: `shopt -q login_shell`
* zsh: `[[ $options[login] = "on" ]]`

But since this is gnarly, let's just avoid it altogether and go the
easy route.

Closes #305, fixes #373, reverts #286

README.md

@@ -167,7 +167,7 @@ easy to fork and contribute any changes back upstream.
     $ echo 'export PATH="$HOME/.rbenv/bin:$PATH"' >> ~/.bash_profile
     ~~~
 
-    **Ubuntu note**: Modify your `~/.profile` instead of `~/.bash_profile`.
+    **Ubuntu Desktop note**: Modify your `~/.bashrc` instead of `~/.bash_profile`.
 
     **Zsh note**: Modify your `~/.zshrc` file instead of `~/.bash_profile`.
 
@@ -177,13 +177,14 @@ easy to fork and contribute any changes back upstream.
     $ echo 'eval "$(rbenv init -)"' >> ~/.bash_profile
     ~~~
 
-    _Same as in previous step, use `~/.profile` on Ubuntu, `~/.zshrc` for Zsh._
+    _Same as in previous step, use `~/.bashrc` on Ubuntu, or `~/.zshrc` for Zsh._
 
-4. Restart your shell as a login shell so the path changes take effect.
-    You can now begin using rbenv.
+4. Restart your shell so that PATH changes take effect. (Opening a new
+   terminal tab will usually do it.) Now check if rbenv was set up:
 
     ~~~ sh
-    $ exec $SHELL -l
+    $ type rbenv
+    #=> "rbenv is a function"
     ~~~
 
 5. Install [ruby-build](https://github.com/sstephenson/ruby-build),