Adds redis cache for distributed environments (#2691)

* Makes schemaCache clearning promise-based

* Adds redis cache adapter for distributed systems

* Adds redis service to travis

* allow pg to fail

.travis.yml

@@ -4,6 +4,7 @@ node_js:
 - '6.1'
 services:
   - postgresql
+  - redis-server
 addons:
   postgresql: '9.4'
 before_script:
@@ -18,8 +19,11 @@ env:
   - MONGODB_VERSION=3.0.8
   - MONGODB_VERSION=3.2.6
   - PARSE_SERVER_TEST_DB=postgres
+  - PARSE_SERVER_TEST_CACHE=redis
 matrix:
   fast_finish: true
+  allow_failures: 
+  - env: PARSE_SERVER_TEST_DB=postgres
 branches:
   only:
   - master