v1.5.0

Lots of new stuff in 1.5.0! We've gotten Rails 5.x support in place, as
well as adding 2 new options. "clustering" will draw a box around models
contained in the same name space, helping to visually organize areas of
concern. "only_models_include_depth" - although awkwardly named - will
restrict the depth of graphing to the supplied value.

• New option of 'clustering' by namespace (#205)
• Support for 'only_models_include_depth' option (#219)
• Added basic support for non-Rails apps (#208)
• Avoid duplicate specializations when using STI with an abstract base class (#211)
• Fixed Ruby 2.1 deprecation warnings (#209)
• Fixes to tests (#210, #213)
• Various documentation fixes (#203, #212)