Don't run `bundle exec rake $RAKE_TASK` if `git diff --exit-code` with 1

MSP-11616

Travis runs all entries in the script section even if earlier entries
exit with 1 and only fails the build as a whole once all entries have
run by &&ing together the exit codes.  Running `bundle exec rake
$RAKE_TASK` if the db/schema.rb is missing updates is unnecessary, so
use `&&` in the script section to stop `bundle exec rake $RAKE_TASK`
from running if `git diff --exit-code` exits with 1.

.travis.yml

@@ -22,8 +22,7 @@ before_script:
   - bundle exec rake db:migrate
 script:
   # fail build if db/schema.rb update is not committed
-  - git diff --exit-code
-  - "bundle exec rake $RAKE_TASK"
+  - git diff --exit-code && bundle exec rake $RAKE_TASK
 
 rvm:
   - '1.9.3'