Address feedback from @wez, Travis

- update Travis set-up to hopefully run Ruby tests (alas, I can't know
  it will work until I push it)
- update query description in README to be more precise
- use long-form license header, hopefully appeasing the Travis linter
- wrap some long lines that the linter was complaining about
- restore fcntl flags on exiting the `RubyWatchman_query` method, also
  ensure xmalloc call frees memory when an exception is raised; this
  required an overall of the error handling in that method
- guard against overflow when peeking at PDU size