No longer upload code coverage to coveralls

We have since moved to codecov and uploading to coveralls is breaking
OS-X builds for py37 [1], so we might as well take this opportunity to drop
it.

[1] https://travis-ci.org/pytest-dev/pytest/jobs/442858038

.travis.yml

@@ -93,12 +93,6 @@ after_success:
       coverage xml --ignore-errors
       coverage report -m --ignore-errors
       bash <(curl -s https://codecov.io/bash) -Z -X gcov -X coveragepy -X search -X xcode -X gcovout -X fix -f coverage.xml -F "${TOXENV//-/,},linux"
-
-      # Coveralls does not support merged reports.
-      if [[ "$TOXENV" = py37 ]]; then
-        pip install coveralls
-        coveralls
-      fi
     fi
 
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