Use build cache for NPM and GYP files

To speed up a build and avoid bombarding
nodejs.org and iojs.org with connections we should
cache what we download via npm and node-gyp.

.travis.yml

@@ -29,6 +29,8 @@ after_success: npm run-script coverage
 
 cache:
   directories:
+    - $HOME/.node-gyp
+    - $HOME/.npm
     - node_modules
 
 notifications:

appveyor.yml

@@ -2,6 +2,10 @@ version: "{build}"
 
 build: off
 
+cache:  
+  - c:\Users\appveyor\.node-gyp
+  - '%AppData%\npm-cache'
+
 environment:
   SKIP_SASS_BINARY_DOWNLOAD_FOR_CI: true
   matrix: