update GETTING_STARTED.md and .gitignore

Add "Prerequisite" section to GETTING_STARTED.md stating that Java 1.7
  should be used.
Add .java_version and .ruby-gemset to .gitignore
add newline to end of .gitignore
Merge pull request #25 from NYULibraries/chore/update-getting-started

Chore/update getting started