Hack: accommodate bad file search path for coverage.xml in CTest

We delete this file because if we don't, CTest tries to submit it
as the Coverage.xml file to CDash, which rejects it.
Actually turn on coverage for travis continuous builds
Consolidating travis cmake script.
Remove duplicative cache value writing for travis
Merge pull request #183 from girder/travis-covg

Hack: accommodate bad file search path for coverage.xml in CTest