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.