(MAINT) Remove Puppet 4x CI testing

As Puppet 5.5 is now the lower bound for supported versions
of Puppet, the testing to ensure compatibility with 4x can
be dropped.

The ruby-pwsh library is not supported on Puppet 4x.

Author: michaeltlombardi

.travis.yml

@@ -39,9 +39,6 @@ matrix:
     -
       env: PUPPET_GEM_VERSION="~> 5.0"
       rvm: 2.4.4
-    -
-      env: PUPPET_GEM_VERSION="~> 4.0" RUBYGEMS_VERSION=2.7.8 BUNDLER_VERSION=1.17.3
-      rvm: 2.1.9
 branches:
   only:
     - master