Elixir 1.7 support

[ma2gedev]

• [x] confirm the library supporting Elixir 1.7
• [x] modify the code to support 1.7

[ma2gedev]

Ebert has finished reviewing this Pull Request and has found:

• 1 possible new issue (including those that may have been commented here).
• 1 fixed issue! 🎉

You can see more details about this review at https://ebertapp.io/github/ma2gedev/power_assert_ex/pulls/17.