Sign hooks before executing with --run flag in Travis

Our new signing logic requires us to sign on the first run. Since this
is running in CI, we're OK signing without manually verifying.

Author: sds

.travis.yml

@@ -22,4 +22,5 @@ before_script:
 
 script:
   - bundle exec rspec
-  - bundle exec bin/overcommit --run
+  - bundle exec overcommit --sign
+  - bundle exec overcommit --run