Add OSX 10.12 and 10.13 (#6760)
Add OSX 10.12 and 10.13 (#6760)

### Problem

We do not test that pants can work in older OSX versions.

### Solution

Add two shards to Travis, one for the earliest version of OSX 10.12 (xcode8.3) and another for the earliest version of 10.13 (9.3).
[Travis OSX version documentation](https://docs.travis-ci.com/user/reference/osx/#using-os-x)

### Acknowledgements

Paired with @dotordogh 
tdyas/pants,jsirois/pants,benjyw/pants,twitter/pants,wisechengyi/pants,tdyas/pants,wisechengyi/pants,twitter/pants,wisechengyi/pants,wisechengyi/pants,tdyas/pants,jsirois/pants,tdyas/pants,tdyas/pants,benjyw/pants,twitter/pants,tdyas/pants,wisechengyi/pants,wisechengyi/pants,twitter/pants,benjyw/pants,twitter/pants,benjyw/pants,twitter/pants,pantsbuild/pants,tdyas/pants,twitter/pants,pantsbuild/pants,pantsbuild/pants,wisechengyi/pants,twitter/pants,twitter/pants,pantsbuild/pants,wisechengyi/pants,pantsbuild/pants,pantsbuild/pants,benjyw/pants,pantsbuild/pants,benjyw/pants,tdyas/pants,jsirois/pants,wisechengyi/pants,benjyw/pants,tdyas/pants