Use debian_gem for bootstrapping Debian Wheezy

The Omnibus installer does not support Debian 7.0 Wheezy yet.
Drop support for Debian 5.0 "Lenny"

It reached EOL already one year ago.
Extract debianoid codename fetching to a method
Use gem installation method for all unknown Debian versions

This covers all unreleased versions.
Use ternary operator instead of one-lines if-else statements
Merge pull request #172 from tmatilai/wheezy

Pepare fails on Debian 7.0 wheezy