Bumps version for Python to 2.7.6

The latest 2.7.X version is 2.7.6, so this bumps `.python-version` to it.

If you're wondering why this is here, it's because Jekyll syntax highlighting currently uses Pygments -- a Python-based syntax highlighting system that is communicated to through a Ruby interface. And it's not compatible with Python 3. So for those using `pyenv`, this ensures that Jekyll will build the site correctly.