Add bower to package.json devDependencies

I ran into the following issue while trying to get setup to work on the
jquery codebase.

  $ npm install
  ...
  > jquery@2.1.0-pre install /private/tmp/jquery
  > bower install

  sh: bower: command not found
  npm ERR! weird error 127
  npm ERR! not ok code 0

The fix is simply to add bower as to devDependencies.