Don't attempt to install PhantomJS on Windows

Instead, assume that phantomjs.exe is already in the user's path. It can
also be overridden on the build.py command line:

  build.py PHANTOMJS=path/to/phantomjs

or with an environment variable:

  set PHANTOMJS=path/to/phantomjs
  build.py
Merge pull request #441 from twpayne/dont-install-phantomjs-on-windows

Do not install PhamtomJS on Windows