chore(grunt): blow away cached node_modules when npm-shrinkwrap.json changes

this replicates the travis setup in grunt from the previous commit

the reason why we duplicate this rather than having just a single place for this code is so that
we can individually time the actions on travis

.travis.yml

@@ -46,6 +46,7 @@ install:
   # Log HTTP requests
   - npm config set loglevel http
   - npm install -g npm@2.5
+  # Instal npm dependcies and ensure that npm cache is not stale (these steps are replicated in lib/grunt/utils.js)
   - diff -q npm-shrinkwrap.json node_modules/npm-shrinkwrap.cached.json; if [[ $? -ne 0 ]]; then echo 'Shrinkwrap changed! Blowing away node_modules.'; rm -rf node_modules; fi
   - time npm install
   - cp npm-shrinkwrap.json node_modules/npm-shrinkwrap.cached.json

lib/grunt/utils.js

@@ -16,7 +16,10 @@ module.exports = {
 
   init: function() {
     if (!process.env.TRAVIS) {
+      // replicated from .travis.yaml
+      shell.exec("diff -q npm-shrinkwrap.json node_modules/npm-shrinkwrap.cached.json; if [[ $? -ne 0 ]]; then echo 'Shrinkwrap changed! Blowing away node_modules.'; rm -rf node_modules; fi");
       shell.exec('npm install');
+      shell.exec('cp npm-shrinkwrap.json node_modules/npm-shrinkwrap.cached.json');
     }
   },