Add PyPy3 build on Travis CI

[redapple]

[codecov-io]

Current coverage is 86.14% (diff: 100%)

Merging #33 into master will decrease coverage by 0.86%

@@             master        #33   diff @@
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  Files             3          3          
  Lines           231        231          
  Methods           0          0          
  Messages          0          0          
  Branches         45         42     -3   
==========================================
- Hits            201        199     -2   
- Misses           29         31     +2   
  Partials          1          1          

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[redapple]

Looks like it's not easy to fix: https://github.com/yyuu/pyenv-virtualenv/issues/188