Update default python version to 2.7.13
Update default python version to 2.7.13

- looks like this was missed when python 2.7.13 was added

[#136306637]

Signed-off-by: Sam Smith <03a07629efb0a02c7bbe7cc340a4243da1ab861a@gmail.com>

cloudfoundry/python-buildpack,cloudfoundry/python-buildpack,cloudfoundry/python-buildpack