Drop puppet_litmus to resolve patron dependency

Commit 6161327 introduced a puppet_litmus
dependency which transitively caused bundler to try to install patron 0.13.4. In
CI, we don't have libcurl headers installed, so patron failed to install its
native extensions.

The problem is caused due to this chain of dependencies:

    puppet_litmus (1.6.1)
      bolt (~> 4.0)
        orchestrator_client (~> 0.7)
          faraday-net_http_persistent (>= 1.0, < 3.0)

When resolving dependencies, bundler first picked faraday-net_http_persistent
2.0.2 and worked backwards to faraday. Faraday 1.9.0 allows 2.x versions of
faraday-* adapters, while faraday 1.10.4 requires 1.x versions. So bundler
backtracked from faraday 1.10.4 to 1.9.0.

    $ BUNDLER_DEBUG_RESOLVER_TREE=1 bundle install
    ...
    attempting faraday-net_http_persistent 2.0.2
    knew: faraday-net_http_persistent >= 2.0.0 depends on Ruby >= 2.6.0
    fact: faraday-net_http_persistent >= 2.0.2, < 2.1.0 depends on faraday-net_http < 3
    fact: faraday-net_http_persistent >= 2.0.0, < 2.2.0 depends on net-http-persistent ~> 4.0
    selected faraday-net_http_persistent 2.0.2
    ...
    attempting faraday 1.9.0
    ...
    fact: faraday >= 1.9.0, < 1.9.1 depends on faraday-patron < 3
    ...
    selected faraday 1.9.0
    ...
    attempting faraday-patron 2.0.2
    ...
    selected faraday-patron 2.0.2
    derived: patron >= 0.4.2
    ...
    selected patron 0.13.4

Since we don't use litmus to test puppet_agent, just remove the gem dependency.

Author: joshcooper

Gemfile

@@ -51,7 +51,6 @@ group :development, :release_prep do
   gem "puppet-blacksmith", '~> 7.0',      require: false
 end
 group :system_tests do
-  gem "puppet_litmus", '~> 1.0',      require: false, platforms: [:ruby, :x64_mingw]
   gem "CFPropertyList", '< 3.0.7',    require: false, platforms: [:mswin, :mingw, :x64_mingw]
   gem "serverspec", '~> 2.41',        require: false
   gem "voxpupuli-acceptance", '~> 3', require: false