Desktop skips & fixes (code review only - do not merge!)

### Do not merge this pull request. It is for code review purposes only

This is blocked by [bug 886741](https://bugzilla.mozilla.org/show_bug.cgi?id=886741) and will require an update once this is resolved and a new version of the Marionette client has been released.

There are a number of fixes for tests running on desktop builds. It allows users to run against the B2G desktop build without having to maintain a separate manifest. It does this by conditionally skipping unsuitable tests and tests that are expected to fail against the desktop build.

This also enables Travis-CI builds for the master branch against the latest mozilla-central nightly B2G desktop build.