Point at home with `~` as `$HOME` might not be exported.

This is true when Jenkins is remotely executing commands on slaves.
Don't automatically load auto-switching

This is surprising in the context of a build slave. Users can still
opt-in to this behavior by sourcing `auto.sh`.
Don't globally load `chruby`.

Some Unixes don't load individual environment startup scripts from
`/etc/profile.d/*`. For consistency it's easiest to just source the
main `/usr/local/share/chruby/chruby.sh` script with `.bashrc`.

We are also being good citizens by not loading `chruby` globally for all
users of the system.
Deploy patched `chruby-exec` that works under non-login shells

The version of `chruby-exec` that ships in `chruby` 0.3.8 does not work
under non-login shells as it assumes `/etc/profile.d/chruby.sh` loaded
the `chruby` function. This file was taken from the following un-merged
PR that fixes the issue:

  https://github.com/postmodern/chruby/pull/250/files