Makes marathon_client gem available to LWRP

Fixes issue where require statement in LWRP would fail due to
unavailable marathon_client gem. Gem is installed during parsing of
LWRP at compile time. Gems have been updated along with their
corresponding files, which includes style and tests.
Update logic for installing marathon_client gem
Fixes rubocop error
Updates Berksfile with correct endpoint
Use Chef resource to install marathon_client gem
Merge pull request #20 from mdsol/feature/install-marathon-client-gem

Makes marathon_client gem available to LWRP