Replace exit with travis_terminate

This is a workaround for the problem that travis seems not to detect
that clang-format has finished and sits waiting for 10 minutes and
fails. test-gen are already using this workaround, cf. diffblue#1300 and diffblue#1331.

Author: Owen Jones

.travis.yml

@@ -28,10 +28,10 @@ jobs:
           echo 'Formatting error! The following diff shows the required changes'
           echo 'Use the raw log to get a version of the diff that preserves spacing'
           cat formatted.diff
-          exit 1
+          travis_terminate 1
         fi
         echo 'No formatting errors found'
-        exit 0
+        travis_terminate 0
       before_cache:
 
 # LOCAL CHANGES: added cbmc/ prefix to both these scripts