(maint) add add'l platforms for acceptance testing

Prior to this commit, we were testing Hiera with a handful of platforms
whose system Ruby was 1.8.7. This led to some acceptance tests failing
when testing against the 'master' branch of Puppet (aka Puppet 4.0).

This commit adds additional platforms, whose system Ruby is 1.9.3+.
Merge pull request #218 from rji/maint/stable/addl-acceptance-platforms

(maint) add add'l platforms for acceptance testing
(fixup) beaker configs should end in .cfg