Removed deprecation warnings in newer rails.

[tdobrovolskij]

As was reported in Issue#793, current version of rails will spam with following warnings:

DEPRECATION WARNING: Passing a template handler in the template name is deprecated. You can simply remove the handler name or pass render :handlers => [:erb]

This fixes it by dropping support for rails 2(it isn't needed anyway, as newer Redmine requires RoR 3.2+).