bump version to 0.2.1

including the following

- Use Kernel.map_size/1 instead of deprecated Map.size/1 #21

bump version to 0.2.0 and now support Elixir version 1.4 or higher

update dependencies for `mix docs` error

```
** (RuntimeError) module PowerAssert.Assertion.Detector was not compiled with flag --docs
    (ex_doc) lib/ex_doc/retriever.ex:151: ExDoc.Retriever.export_docs?/1
    (ex_doc) lib/ex_doc/retriever.ex:135: ExDoc.Retriever.get_module/2
    (elixir) lib/enum.ex:1314: Enum."-map/2-lists^map/1-0-"/2
    (ex_doc) lib/ex_doc/retriever.ex:115: ExDoc.Retriever.docs_from_modules/2
    (ex_doc) lib/ex_doc.ex:73: ExDoc.generate_docs/3
    (ex_doc) lib/mix/tasks/docs.ex:131: anonymous fn/6 in Mix.Tasks.Docs.run/3
    (elixir) lib/enum.ex:1925: Enum."-reduce/3-lists^foldl/2-0-"/3
    (ex_doc) lib/mix/tasks/docs.ex:130: Mix.Tasks.Docs.run/3
```

bump version to 0.1.1

bump version to 0.1.0

bump version to 0.0.9

bump version to 0.0.8

bump version to 0.0.7

bump version to 0.0.6

bump version to 0.0.5