Change Travis runs to use overcommit --run

This feature was recently added and allows us to run all pre-commit
hooks against this repository. Changing our Travis runs to us it gives
us two benefits:

  * We can dog food the feature itself to ensure it continues working
  * We can add additional tests that this repo should run by adding
    Overcommit hooks, which is a good incentive to add more hooks

This also means that our pre-commit process and our actual CI runs are
now at parity with one another, so there should be no discrepancy
between the two.

Change-Id: I632b67ed5fbfe15824154f8de1bbfd95fe167171
Reviewed-on: http://gerrit.causes.com/45338
Tested-by: jenkins <jenkins@brigade.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>

Author: sds

.travis.yml

@@ -14,4 +14,4 @@ before_script:
 
 script:
   - bundle exec rspec
-  - bundle exec rubocop
+  - bundle exec bin/overcommit --run