Added Travis CI OSX Image and Install cordova@latest (#59)

diff --git a/.travis.yml b/.travis.yml
index c912dd7..09f5ab7 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,4 +1,5 @@
 language: objective-c
+osx_image: xcode9.4
 sudo: false
 
 env:
@@ -11,6 +12,7 @@
   - nvm install $TRAVIS_NODE_VERSION
 
 install:
+  - npm install -g cordova@latest
   - npm install
 
 script: