Fixed Travis permission problem

["Driesprong, Fokko" <fo...@driesprong.frl>]

Hi all,

I've hotfixed the CI permission problem:

Exception:
Traceback (most recent call last):
  File "/home/travis/build/apache/incubator-airflow/.tox/py35-backend_mysql/lib/python3.5/site-packages/pip/basecommand.py",
line 215, in main
    status = self.run(options, args)
  File "/home/travis/build/apache/incubator-airflow/.tox/py35-backend_mysql/lib/python3.5/site-packages/pip/commands/wheel.py",
line 199, in run
    if not wb.build():
  File "/home/travis/build/apache/incubator-airflow/.tox/py35-backend_mysql/lib/python3.5/site-packages/pip/wheel.py",
line 749, in build
    self.requirement_set.prepare_files(self.finder)
  File "/home/travis/build/apache/incubator-airflow/.tox/py35-backend_mysql/lib/python3.5/site-packages/pip/req/req_set.py",
line 380, in prepare_files
    ignore_dependencies=self.ignore_dependencies))
  File "/home/travis/build/apache/incubator-airflow/.tox/py35-backend_mysql/lib/python3.5/site-packages/pip/req/req_set.py",
line 620, in _prepare_file
    session=self.session, hashes=hashes)
  File "/home/travis/build/apache/incubator-airflow/.tox/py35-backend_mysql/lib/python3.5/site-packages/pip/download.py",
line 809, in unpack_url
    unpack_file_url(link, location, download_dir, hashes=hashes)
  File "/home/travis/build/apache/incubator-airflow/.tox/py35-backend_mysql/lib/python3.5/site-packages/pip/download.py",
line 715, in unpack_file_url
    unpack_file(from_path, location, content_type, link)
  File "/home/travis/build/apache/incubator-airflow/.tox/py35-backend_mysql/lib/python3.5/site-packages/pip/utils/__init__.py",
line 599, in unpack_file
    flatten=not filename.endswith('.whl')
  File "/home/travis/build/apache/incubator-airflow/.tox/py35-backend_mysql/lib/python3.5/site-packages/pip/utils/__init__.py",
line 482, in unzip_file
    zipfp = open(filename, 'rb')
PermissionError: [Errno 13] Permission denied:
'/home/travis/.wheelhouse/Flask_Admin-1.5.0-py3-none-any.whl'

I've fixed this now by explicitly removing the file:
https://github.com/apache/incubator-airflow/pull/2993/files

I know this isn't the nicest solution to fix this, but I don't have any
access to Travis to wipe the cache or so. We can revert this once this file
is evicted from the cache. If there are any questions, please let me know.

Cheers, Fokko