Deprecate Python 2.5

Neither of our testing tools (Travis and tox) support Python 2.5 anymore.  This means that continuing to claim support is either (a) onerous or (b) a lie.  Based on our data, almost no one is currently using Python 2.5 in production and none of them have updated their bindings this year.  

I'd like to officially deprecate Python 2.5 without doing anything to intentionally break it.  For the time being, we won't refactor the code to use `with`, `except ... as` or libraries unavailable in 2.5.  We simply won't commit to maintaining that compatibility in the future.

@danielhfrank @kyleconroy does this seem fair given the context from our email thread?