Add Roboto to the `$helvetica` font stack for Android support

Most Android devices do not have either Helvetica Neue or Helvetica available,
but as of Android 4.0 "Ice Cream Sandwich" there is a look-alike font called
Roboto that serves as a decent stand-in for the (better) Helvetica fonts.
Merge pull request #274 from jmmcduffie/add-roboto-to-helvetica-font-stack

Add Roboto to the `$helvetica` font stack for Android support