travis: Use the default osx image.

[rillian]

XCode 7.3 (based on macOS 10.11) is out of support, and cannot complete tests because the brew upgrade step has so many packages to build. Use the default image instead, which is currently macOS 10.13 and Xcode 9.4.1.

This seems to resolve the recent travis test failures on macOS.

[rillian]

Note that the linux 2.7 python job has started failing in the meantime, so tests still won't be green with this commit. I suspect that can be fixed by removing coverage from that job as was done for the jython job, if that's a reasonable approach?

[codecov-io]

Codecov Report

Merging #76 into master will decrease coverage by 0.09%. The diff coverage is n/a.

@@            Coverage Diff            @@
##           master      #76     +/-   ##
=========================================
- Coverage   87.24%   87.14%   -0.1%     
=========================================
  Files          26       27      +1     
  Lines         980      988      +8     
=========================================
+ Hits          855      861      +6     
- Misses        125      127      +2

Impacted Files	Coverage Δ
pyoracc/__init__.py	75% <0%> (ø)

---

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[rillian]

Adding Python 3.7 on Linux turned out to be a bridge too far, but the additional commit have all tests passing again. @jayanthjaiswal, @raquel-ucl what do you think?

[jayanthkmr]

@rillian Great work! Thanks. Removing coverage won't hurt much.

[raquelalegre]

Thanks @rillian!

[rillian]

Thanks for reviewing! I don't have commit access. Could someone please push the merge button so we can make the badge green? :)

[giordano]

There you go!

[rillian]

danke!