Make flake8 mandatory on travis

The travis build will fail if flake8 finds errors. Since not all code is
cleaned yet we ignore a couple of files in `setup.cfg`.

We'll gradualy clean the code remove the excluded files from `setup.cfg`.

See #669.

.travis.yml

@@ -11,10 +11,6 @@ env:
     - TOX_ENV=docs
     - TOX_ENV=flake8
 
-matrix:
-    allow_failures:
-        - env: TOX_ENV=flake8
-
 install:
     - travis_retry sudo apt-get update
     - travis_retry sudo apt-get install -qq bash-completion mp3gain

setup.cfg

@@ -6,3 +6,7 @@ logging-clear-handlers=1
 # F401  module imported but unused
 # E231  missing whitespace after ',' (used to align visually)
 ignore=F401,E241
+
+# List of files that have not been cleand up yet. We will try to reduce
+# this with each commit
+exclude=test/*,beetsplug/*,beets/autotag/hooks.py,beets/autotag/__init__.py,beets/autotag/match.py,beets/autotag/mb.py,beets/dbcore/db.py,beets/importer.py,beets/library.py,beets/plugins.py,beets/ui/commands.py,beets/ui/__init__.py,beets/ui/migrate.py,beets/util/artresizer.py,beets/util/bluelet.py,beets/util/confit.py,beets/util/enumeration.py,beets/util/functemplate.py,beets/util/pipeline.py