.travis/after_success.sh: upload combined coverage data to codecov.io for each build
.travis/after_success.sh: upload combined coverage data to codecov.io for each build

Previously we were using a separate Travis build that only run coverage with a single Python version (3.5).
Now, we run coverage with all python versions, and upload a combined coverage data to Codecov.io.

fonttools/fonttools,googlefonts/fonttools