Add contribution guidelines + Rubocop instructions
Merge pull request #12 from dpla/rubocop-instructions

Add contribution guidelines + Rubocop instructions
adding travis config
Make CI work

Removes an errant require to krikri/ldp (which is still in another
branch), and introduces new steps to `ci` task:

  - marmotta start/stop (note that stop is not always successful)
  - jetty:clean (creates jetty directory)
  - jetty:config (to copy solr config into new jetty instance)