restrict NPM installs based on installed node version

attempt to fix build on Node 0.8, where NPM was attempting to install
Bower 1.3.8, which is incompatible

npm/npm#3662 (comment)

Author: afeld

.travis.yml

@@ -5,4 +5,5 @@ node_js:
   - "0.10"
   - "0.11"
 before_install:
+- npm config set engines-strict true
 - npm install grunt-cli bower -g