chore(travis): skip build on deployment job when from Pull Request

scripts/travis/build.sh

@@ -46,8 +46,13 @@ case "$JOB" in
     grunt test:travis-protractor --specs="$TARGET_SPECS"
     ;;
   "deploy")
-    grunt package
-    grunt compress:firebaseCodeDeploy
+    # we never deploy on Pull requests, so it's safe to skip the build here
+    if [[ $TRAVIS_PULL_REQUEST != 'false' ]]; then
+      grunt package
+      grunt compress:firebaseCodeDeploy
+    else
+      echo "Skipping build because Travis has been triggered by Pull Request"
+    fi
     ;;
   *)
     echo "Unknown job type. Please set JOB=ci-checks, JOB=unit, JOB=deploy or JOB=e2e-*."