Add support for parallel builds
Add support for parallel builds

From https://docs.coveralls.io/parallel-build-webhook and
https://docs.coveralls.io/api-reference, all that is needed is to
pass-through an environment variable to a field named 'parallel' in
the job payload.

Signed-off-by: Iustin Pop <acf83a5274f31a43af6b914614365999469c1a31@k1024.org>

okkez/coveralls-lcov