(maint) use a more recent version of beaker

The Gemfile used for Hiera acceptance testing pins to an ancient version
of Beaker. With this commit, we pin this to the current version of
Beaker, 1.17.x, recommended for use in all acceptance testing.
Merge pull request #209 from rji/maint/stable/use-recent-beaker

(maint) use a more recent version of beaker