Fix unclickable Zen Flat buttons in Gmail web

First reported in #2 as an inliner problem, but this had nothing to do with the
inlining.
Merge pull request #8 from sendwithus/fix-zen-flat-buttons-gmail-web

Fix unclickable Zen Flat buttons in Gmail web
Merge branch 'master' into premailer-fixes

* master:
  Fix unclickable Zen Flat buttons in Gmail web