Add conditional sourcing.

It depends on the used distro, which file needs
to be sourced to get __gi_ps1

- First for Fedora >= 17
- Second for CentOS and older Fedora
Tweak rvm prompt into separate line.

Also, the native rvm-prompt command is used instead of
some custom scripting fu.
Remove custom rvm prompt function.
Refactor git prompt helpers - check git only once.

Move duplicated conditional for checking if the current
directory is a git repo from the respective helper function
into one central place: before building PS1