Travis: more specific Python 2.7 release number

Failures were cropping up due to a combination of a Travis problem where
the Python version was reverted to Python 2.7.6:
https://github.com/travis-ci/travis-ci/issues/8097

And a pylast update that called a function only available on Python
2.7.9 and above, discussed here:
https://github.com/pylast/pylast/issues/201

I hope this fixes it...

.travis.yml

@@ -9,7 +9,7 @@ env:
 
 matrix:
     include:
-        - python: 2.7
+        - python: 2.7.13
           env: {TOX_ENV: py27-cov, COVERAGE: 1}
         - python: 2.7_with_system_site_packages
           env: {TOX_ENV: py27-test}
@@ -25,7 +25,7 @@ matrix:
         #   - env: {TOX_ENV: pypy-test}
         - python: 3.4
           env: {TOX_ENV: py34-flake8}
-        - python: 2.7
+        - python: 2.7.13
           env: {TOX_ENV: docs}
 # Non-Python dependencies.
 addons: