(maint) Update ruby sitelibdir path for PE 3

Previously pe-puppetdb-terminus was installed to the ruby 1.8 sitelibdir for
pe-ruby1.8. In PE 3, the ruby sitelibdir is the same for all dists. This commit
sets it as a constant and uses that path to install the terminus files.
Use RbConfig on PE, which has ruby 1.9 and where Config is deprecated.
(maint) We have @plibdir, let's use it

The puppetdb Rakefile defines some awesome instance variables, like @plibdir,
which points to the puppet ruby sitelibdir. We should totally use this in the
terminus.install template instead of switching on PE and using hard coded
paths.
Merge pull request #501 from haus/misc_pe_puppetdb_packaging_fixups

Misc pe puppetdb packaging fixups