Speed up Travis builds

- Install firefox only when required (vs addon before)
- Remove benchmark tests - output is meaningless on Travis VMs
- //ize Chrome and FF builds (JS)
- Updated FF to v29.0

Opened questions: 
- is [this](https://github.com/vicb/angular.dart/compare/angular:master...vicb:0507-travis?expand=1#diff-d423681a1a5ceb4c78c390f0d30bb343R11) relevant for Travis ? 
- what about using `fast_finish: true` in the matrix config, ie the build would stop if any of the sub-builds fail ?