Make the flake8 job use a separate cache

The CACHE_NAME environment variable hack is recommended by
https://docs.travis-ci.com/user/caching/

Also, AFAIU the before_cache: rm pip/debug.log is needed only if you
manually try to cache the ~/.cache/pip directory, and shouldn't be
needed for cache: pip.

Also I like 2-space indents in YAML files so there.

.travis.yml

@@ -1,28 +1,27 @@
 language: python
 python:
-    - 2.7
-    - 3.5
-    - 3.6
-    - 3.7
-    - 3.8
-    - pypy
+  - 2.7
+  - 3.5
+  - 3.6
+  - 3.7
+  - 3.8
+  - pypy
 jobs:
   include:
     - name: flake8
       install: pip install flake8
       script: flake8 src setup.py
       after_success:
+      env: CACHE_NAME=flake8
 install:
-    - pip install -U pip setuptools
-    - pip install -U coverage coveralls
-    - pip install -U -e .[test,docs]
+  - pip install -U pip setuptools
+  - pip install -U coverage coveralls
+  - pip install -U -e .[test,docs]
 script:
-    - coverage run -m zope.testrunner --test-path=src
-    - coverage run -a -m sphinx -b doctest -d docs/_build/doctrees docs docs/_build/doctest
+  - coverage run -m zope.testrunner --test-path=src
+  - coverage run -a -m sphinx -b doctest -d docs/_build/doctrees docs docs/_build/doctest
 notifications:
-    email: false
+  email: false
 cache: pip
-before_cache:
-    - rm -f $HOME/.cache/pip/log/debug.log
 after_success:
   - coveralls