Revert "don't run tests from /build"

This reverts commit 1687892d63.

This is no longer necessary with the fix in the previous commit.

.travis.yml

@@ -9,9 +9,7 @@ install:
 - pip install coverage coveralls flake8 flake8-bugbear mypy
 - pip install -e .
 script:
-- sudo mkdir -p /black
-- sudo mount --bind $(pwd) /black
-- coverage run /black/tests/test_black.py
+- coverage run tests/test_black.py
 - if [[ $TRAVIS_PYTHON_VERSION == '3.6' ]]; then mypy black.py tests/test_black.py; fi
 - if [[ $TRAVIS_PYTHON_VERSION == '3.6-dev' ]]; then flake8 black.py tests/test_black.py; fi
 after_success: