Use the freshly built ruby for running fpm

Using the recently built Ruby means we don't have to worry about Ohai
data and Gem's bindir paths being correct. Thus we also avoid the need
to install system ruby, normally on separate Chef run.

The easiest way to set GEM_HOME for installing and running fpm seemed to
be to use environment variable.
Update the README
Update the CHANGELOG
Merge pull request #2 from tmatilai/use-built-ruby-for-fpm

Use the built ruby for running fpm