Install sqlite3 from gems on EL during acceptance testing

When installing Puppet 3.x in acceptance tests, the centos5 machine has
1.8.7 installed as a dependency. This means the ruby-sqlite3 package
doesn't work correctly, as it's meant for use with Ruby 1.8.5. Now we
just install the devel package, and install sqlite3 as a gem.

We also always install rubygems, as it was missing on certain platforms.