Allow adding elements to the ruby compile $PATH

When compiling Ruby 2.1.1 on a recent Mavericks system with Homebrew installed, it fails to compile because the openssl vendored into ruby seems to not compile. We've worked around this issue by installing openssl via homebrew, and then adjusting the `$PATH` to include `/usr/local/bin` (so the build can find the openssl configuration). This requires support from puppet-rbenv, since it sets a minimal `$PATH` that doesn't include `/usr/local/bin`. 

Does this seem like something you can include? If so, feel free to pull it in (-: