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