travis-ci: send coverage report to coveralls

https://github.com/coveralls-clients/coveralls-python/blob/1.7.0/README.rst
https://github.com/z4r/python-coveralls/blob/2.9.1/README.rst#travisyml

https://github.com/fphammerle/freesurfer-volume-reader/issues/8

.travis.yml

@@ -41,3 +41,7 @@ install:
 script:
 - pipenv run pylint freesurfer_volume_reader tests/*
 - pipenv run pytest --cov=freesurfer_volume_reader --cov-report=term-missing --cov-fail-under=100
+
+after_success:
+- pip install coveralls
+- coveralls