Updated install doc to remove Python 2.5 as target

According to https://github.com/cobrateam/splinter/commit/75c0ec1b2a115eb361ccd5228ba0ddfc2f9e0984 support for Python 2.5 has been dropped. Here I've also generalized Python 3.0 to 3.x ... which might now be supported?
Support for Python 2.7+ only
Merge pull request #329 from mdxs/patch-1

Updated install doc to remove Python 2.5 as target