fix test on Elixir version 1.4 or later

[ma2gedev]

close #8

[ma2gedev]

Ebert has finished reviewing this Pull Request and has found:

• 33 possible new issues (including those that may have been commented here).
• 2 fixed issues! 🎉

You can see more details about this review at https://ebertapp.io/github/ma2gedev/power_assert_ex/pulls/12.