Don't have Travis run pre-commit on Python 2.6

(pre-commit no longer supports 2.6)

.travis.yml

@@ -12,4 +12,4 @@ install:
   - pip install tox-travis pre-commit
 script:
   - tox
-  - pre-commit run --all-files
+  - if [[ $TRAVIS_PYTHON_VERSION != 2.6 ]]; then pre-commit run --all-files; fi

requirements.txt

@@ -4,6 +4,5 @@ pytest
 pytest-cov
 sphinx
 tox
-pre-commit
 
 -e .