puppet5.5: restore support and tests for puppet5.5

* Github actions
  * Manually specify matrix for debian and puppet5.5
  * use ruby 2.5 by default for spec tests
  * drop lint/rubocop tests as puppetlabs can do this
* Gemfile
  * drop dependency of puppet-module-posix-* to '~> 0.0' for spec tests
  * drop puppet-module-win-*
  * use default puppet version of '~> 5.5.0'
* fixtures
  * force version of puppetlabs-puppet_agent so we can use puppet5

.fixtures.yml

@@ -1,8 +1,10 @@
 fixtures:
   repositories:
     facts: 'https://github.com/puppetlabs/puppetlabs-facts.git'
-    puppet_agent: 'https://github.com/puppetlabs/puppetlabs-puppet_agent.git'
+    puppet_agent:
+      repo: 'https://github.com/puppetlabs/puppetlabs-puppet_agent.git'
+      ref: 4.4.0
     provision: 'https://github.com/puppetlabs/provision.git'
   symlinks:
     stdlib: "#{source_dir}"
-    test:   "#{source_dir}/spec/fixtures/test"
+    test:   "#{source_dir}/spec/fixtures/test"