Removed deprecation warnings in newer rails.

As was reported in [Issue#793](https://github.com/backlogs/redmine_backlogs/issues/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+).