nunjucksLoader.getSource function not properly handling Windows paths

Windows paths were matching the Regex on line 58, so I've fixed this to require a module to have 2 or more characters. This fixed the windows issue and it also still resolves all modules (albeit 2+ characters).
Merge pull request #68 from MichaelMackus/patch-2

nunjucksLoader.getSource function not properly handling Windows paths