(Maint) Use absolute path for puppet in Gemfile

This allows the use of binstubs to execute puppet from another working
directory.
Merge branch '2.7.x' into 3.1.x

* 2.7.x:
  (Maint) Use absolute path for puppet in Gemfile
Merge branch '3.1.x'

* 3.1.x:
  (Maint) Use absolute path for puppet in Gemfile