Update Travis-CI configuration to build es-private-fields branch

Author: kpfleming

.travis.yml

@@ -15,12 +15,7 @@ matrix:
 
 branches:
   only:
-  - master
-  - release-2.7
-  - release-2.8
-  - release-2.9
-  - release-3.0
-  - release-3.1
+  - es-private-fields
 
 install:
   - npm uninstall typescript --no-save