travis: Move flake8 to its own build

.travis.yml

@@ -7,16 +7,16 @@ env:
   # TODO Activate when Travis has PyPy 2.x
   #- TOX_ENV=pypy
   - TOX_ENV=docs
+  - TOX_ENV=flake8
 
 install:
   - "wget -O - http://apt.mopidy.com/mopidy.gpg | sudo apt-key add -"
   - "sudo wget -O /etc/apt/sources.list.d/mopidy.list http://apt.mopidy.com/mopidy.list"
   - "sudo apt-get update"
   - "sudo apt-get install python-all-dev libffi-dev libspotify-dev"
-  - "pip install flake8 tox"
+  - "pip install tox"
 
 script:
-  - "flake8 $(find * -iname '*.py')"
   - "tox -e $TOX_ENV"
 
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