Remove .egg-info/ to allow wheel installation

tox now invokes pip as a python module instead of through its entry
point. "python -m" adds the current directory to the PYTHONPATH, picking
up the .egg-info/ metadata directory, tricking pip into thinking that
the package is already installed (and thus not installing the wheel).
Deleting the metadata directory fixes this.

.travis.yml

@@ -29,6 +29,7 @@ matrix:
         env: TOXENV=dist
         script:
           - python setup.py bdist_wheel
+          - rm -r djangorestframework.egg-info  # see #6139
           - tox --installpkg ./dist/djangorestframework-*.whl
           - tox  # test sdist