Updates to travis yml (#81)

* Add osx as travis target
* Remove caching -- we don't have a large node_modules and it will catch more issues.
* Only run travis on master
* Build with npm 4 for now due to an issue on osx that's being investigated... more info to follow.

.travis.yml

@@ -1,18 +1,29 @@
-dist: trusty
+matrix:
+  include:
+    - os: linux
+      dist: trusty
+    - os: osx
+
 language: node_js
+
 node_js:
   - 8
+
 addons:
   apt:
     packages:
       - libsdl2-dev
       - fcitx-libs-dev
       - libibus-1.0-dev
-cache:
-  directories:
-    - node_modules
+
 script:
   - npm install
   - npm run build
   - npm run build:bytecode
   - npm run build:native
+
+before_install: if [[ `npm -v` != 4* ]]; then npm i -g npm@4; fi
+
+branches:
+  only:
+  - master