coveralls: another take on getting coverage to work

- travis: use gcov rather than lcov
- travis: don't use after_success as it seems to get ignored unless we
  deploy
- travis: use cpp-coveralls rather than coveralls-lcov
- travis, cmake: remove WITH_COVERAGE flag, use instead WITH_DEBUG and
  WITH_PROFILING
- readme: use coveralls badge as per suggestions from their site
- cmake: fix incorrect sprint number