Add .travis.yml to enable continuous integration testing

I'd like to validate that this code works in some other environment.
Furthermore, I'd like to easily continue supporting this application
on both Ruby 1.9.3 and 2.0.0.

Setup continuous integration using Travis-CI.org.
Attempt to fix intermittent failing spec for dosing exceptions

This spec occasionally fails:

  1) Feeding Dosing records exceptions as failures
     Failure/Error: dose.finished_at.to_f.should be_within(0.2).of(DateTime.now.to_f)
       expected 1374982365.680259 to be within 0.2 of 1374982365.9335904
     # ./spec/features/pump_control_spec.rb:116:in `block (3 levels) in  <top (required)>'

Try to address the problem, at least partially, by collecting the
expected failure time more precisely.  This functionality is just plain
hard to test because it runs in a different thread and the main
application code doesn't join this thread.
Add Travis-CI web badge to README