Only run wpcept against WordPoints stable

It doesn’t make sense to run it against them against the development
version, because they will inevitably fail as there will be no updates
for WordPoints. Possibly we’ll change this in the future by adding some
sort of workaround.

See #6

.travis.yml

@@ -71,7 +71,7 @@ script:
     - phpunit-ajax
     - phpunit-ms-ajax
     - phpunit-ms-network-ajax
-    - wpcept-run
+    - if [[ $WORDPOINTS_VERSION != master ]]; then wpcept-run; fi
 
 after_script:
     - source $DEV_LIB_PATH/travis/after_script.sh