Don't install the package in edit mode to fix OptionConflictError

The entrypoints package (https://github.com/takluyver/entrypoints/) was introduced in flake8 3.7.

Unfortunately it will detect entrypoints multiple times when the plugin is installed in editable mode
because of this line: https://github.com/takluyver/entrypoints/blob/master/entrypoints.py#L192

The '-' is not in the directory name when you install a package in edit mode so when the python
path contains duplicate paths to the same module flake8 will instantiate the plugin multiple times
as well.

.travis.yml

@@ -7,7 +7,7 @@ python:
   - 'pypy3'
 cache: pip
 install:
-  - pip install -e .[test]
+  - pip install .[test]
 script:
   - flake8 *.py
   - pytest run_tests.py