Only run linting tests for one Python version

This creates a specific job with the `FLAKE8` variable set, rather than
a setting up a 4×2 matrix. This means we create a total of 5 jobs now
rather than 8 jobs.

Part of https://github.com/whipper-team/whipper/pull/433

Signed-off-by: Frederik “Freso” S. Olesen <freso.dk@gmail.com>

.travis.yml

@@ -14,7 +14,11 @@ cache: pip
 
 env:
     - FLAKE8=false
-    - FLAKE8=true
+
+jobs:
+  include:
+  - python: 3.5
+    env: FLAKE8=true
 
 install:
     # Dependencies