Ensure `tar` is available for native extension compilation

Many gems ship with native extensions that assume tar will be available
in the PATH during the build process. Although we would hope these native 
extensions respect environment variables like `TAR` or `PROG_TAR` it’s 
not always the case. :-(
Merge pull request #303 from opscode/schisamo/tar-in-devkit

Ensure `tar` is available for native extension compilation