Move 'deg2rad' from monkeypatch to private method

In Ruby 2.1.2, we were encountering issues where `angle` was a BigDecimal, and `deg2rad` was defined as a method on Float, so we were getting an undefined method error. This commit moves `deg2rad` into a private method on Gruff::Base that takes an argument that can be any numeric type.

Similar issue: https://github.com/topfunky/gruff/issues/97