Skip coverage of vendored and gemmed assets

I'm digging the instrumentation from #31, but my reports are full of
stats on gemmed assets (e.g.
`.rbenv/versions/2.0.0-p0/lib/ruby/gems/2.0.0/gems/backbone-on-rails-1.0.0.0/vendor/assets/javascripts/`).

This patch omits non-app assets by default (i.e. any in gems or
`vendor/assets`).