Fix deploy step

Deploy only for one Python env. If we don't restrict here then Travis
will try to upload to PyPi multiple times (once for each Python env)
and will fail.

This change creates a universal `bdist_wheel`.

Signed-off-by: Nicolas Bock <[email protected]>

Author: nicolasbock

Diff for: .travis.yml

@@ -30,4 +30,5 @@ deploy:
   skip_cleanup: false
   on:
     tags: true
+    python: "3.6"
     branch: master

Diff for: setup.cfg

@@ -0,0 +1,2 @@
+[bdist_wheel]
+universal = 1