fix(travis): install correct npm version

Author: geraintwhite

.travis.yml

@@ -6,6 +6,13 @@ node_js:
   - '0.11'
   - '0.12'
 
+before_install:
+  - npm i -g npm@^2.0.0
+
+before_script:
+  - npm prune
+
+
 script:
   - npm run coverage