Add LWRP for chruby_exec and chruby_gem.

This commit adds the LWRP for running chruby-exec and gem commands in
the context of a supplied Ruby version.

Additional changes:
- Default to the latest, stable version of chruby;
- Default to the latest, stable version of Ruby 1.9.3;
- Install default gems using LWRP for all rubies;
- Attribute for specifying installation prefix.