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
remove `rbenv install` example; clarify manual compile

It can be confusing if people mistake the example as instructions to
install ruby-build with.
tweak installation instructions

Provide an `rbenv install` example and explanation lower down, so that
both folk that are following git install and Homebrew install will
finish at the same section and learn how to use ruby-build.

Also remove step 6 of manual install process: `rbenv rehash`. It's
unnecessary at that point, even if they already did `rbenv install`
(which rehashes automatically).

Fixes #455, closes #456