Use $is_pe for PE determination

...rather than the custom logic that was used previously. Using a
uniform canon fact across all modules that need to differentiate between
PE and POSS makes unusual situations (such as having a missing
/opt/puppet/pe_version file) more, well, uniform.
Bump version requirement for puppetlabs/stdlib

The is_pe fact was first defined in stdlib in 3.1.0. We want to use that
fact for making the "PE or Not PE" determination in params, so we need
to require a stdlib at least that new. Note that at the time of commit
the current version of stdlib is 4.1.0, so we're not exactly requiring
bleeding edge.
Merge pull request #122 from reidmv/use_is_pe

Use $is_pe for PE determination