Skip browser testing on Sauce Labs for PRs

This just doesn't seem to work on Travis CI -- there seems to be no currently supported way to safely share the login credentials on PR builds

.travis.yml

@@ -18,7 +18,9 @@ install:
 script:
   - (cd packages/library && yarn run build:starterkit)
   - (cd packages/library && yarn test)
-  - (cd packages/library && yarn run test:sauce)
+  - if [ $TRAVIS_PULL_REQUEST == "false" ]; then
+      (cd packages/library && yarn run test:sauce);
+    fi
   - (cd packages/builder && yarn run build)
   - (cd packages/builder && yarn test)
 after_success: