(MAINT) Bump travis puppet4 cell to Ruby 2.1.9

The README specifies a minimum Ruby version of 2.1.9, and we only
relaxed the gemspec requirement to 2.1.0 in PDOC-259 to support some
puppet_agent module testing. Skipping 2.1.0 for testing allows us to
bump our dev-only dependency on Beaker to a reasonably modern range.
Since there is no runtime dependency on Beaker, this shouldn't break
downstream users on Ruby 2.1.0.

Author: scotje

.travis.yml

@@ -14,7 +14,7 @@ before_install:
 matrix:
   fast_finish: true
   include:
-  - rvm: 2.1.0
+  - rvm: 2.1.9
     env: PUPPET_GEM_VERSION="~> 4" CHECK=spec
   - rvm: 2.4.1
     env: PUPPET_GEM_VERSION="~> 4" CHECK=spec