chore: only run example tests if code has changed (#3773)

No need to run example tests if we've just changed docs etc.

.travis.yml

@@ -291,7 +291,7 @@ jobs:
       script:
         # Travis lets scripts continue even if previous steps fail so need to use &&: https://github.com/travis-ci/travis-ci/issues/1066
         - npm run configure-examples &&
-          npm run test -- --scope=example* --concurrency=1
+          npm run test -- $RUN_SINCE --scope=example* --concurrency=1
 
     - stage: release-rc
       # only run on changes to master