Enable cache on travis

.travis.yml

@@ -21,6 +21,13 @@ before_install:
 - npm install -g gulp
 - npm install
 
+cache:
+  directories:
+  - node_modules
+  - app/node_modules
+  - $HOME/.electron
+  - $HOME/.cache
+
 script:
 - npm run travis
 notifications:
@@ -28,4 +35,4 @@ notifications:
     urls:
       - https://zulip.org/zulipbot/travis
     on_success: always
-    on_failure: always
+    on_failure: always