Try tests with slow variant

The tests failed at [an unexpected place] with Travis. I can't reproduce
the failure locally. This is the best way I can think of to debug the
situation.

[an unexpected place]: https://github.com/jakar/vim-python-pep8-indent/blob/dd18c0397b4d2269b3346f5f8ec3100c33efae2b/spec/indent/indent_spec.rb#L181

Author: jakar

.travis.yml

@@ -15,5 +15,5 @@ before_script:
   - pip install --user vim-vint
   - vim --version
 script:
-  - make test
+  - make test_slow
   - vint **/*.vim