Excludes option no longer excludes about tools

[ebidel]

There's still a single polymer.html left in the vulcanized output. Repo:

git clone [email protected]:Polymer/docs.git
npm install
grunt vulcanize

check the output in /elements/common_elements.vulcanized.html

Our grunt-vulcanize config is:

vulcanize: {
  options: {
    excludes: {
      imports: [
        "polymer.html$" 
      ]
    },
    strip: true,
    csp: false,
    // inline: true
  },
  build: {
    files: {
      'elements/common_elements.vulcanized.html': 'elements/common_elements.html'
    },
  }
},