Updated install doc to remove Python 2.5 as target
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?
cobrateam/splinter,myself659/splinter,bmcculley/splinter,myself659/splinter,cobrateam/splinter,gjvis/splinter,cobrateam/splinter,underdogio/splinter,underdogio/splinter,lrowe/splinter,lrowe/splinter,gjvis/splinter,nikolas/splinter,nikolas/splinter,objarni/splinter,drptbl/splinter,drptbl/splinter,bmcculley/splinter,underdogio/splinter,drptbl/splinter,bmcculley/splinter,nikolas/splinter,myself659/splinter,objarni/splinter,objarni/splinter,gjvis/splinter,lrowe/splinter