chore(travis): add npm/install-dependencies.sh step in preparation for caching

tools/npm/install-dependencies.sh

@@ -0,0 +1,16 @@
+#!/bin/bash
+
+set -e
+
+SHRINKWRAP_FILE=npm-shrinkwrap.json
+SHRINKWRAP_CACHED_FILE=node_modules/npm-shrinkwrap.cached.json
+
+if diff -q $SHRINKWRAP_FILE $SHRINKWRAP_CACHED_FILE; then
+  echo 'No shrinkwrap changes detected. npm install will be skipped...';
+else
+  echo 'Blowing away node_modules and reinstalling npm dependencies...'
+  rm -rf node_modules
+  npm install
+  cp $SHRINKWRAP_FILE $SHRINKWRAP_CACHED_FILE
+  echo 'npm install successful!'
+fi