Set Cairo compatibility

[CiaranOMara]

This PR resolves dependencies for #1339, closes #1320 and also closes #1324.

[bjarthur]

the docs have repeatedly failed to pass CI with his PR. i've restarted multiple times and get the same "stalled build" error. does this work for you locally?

worth noting that CategoricalArrays is now on v0.7, and Gadfly/Project.toml only lists 0.5.

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@@            Coverage Diff            @@
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  Hits              ?     3551           
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[CiaranOMara]

My Travis build also stalled in the same spot.

However, the documentation did build locally.

Albeit, it took a while. I've prefixed the build with travis_wait (I may have put it in the wrong spot), which should let it keep running.

[Mattriks]

Does your local build of the docs use Compose master?

[CiaranOMara]

No it did not. It used the 0.7.4 release.

[CiaranOMara]

The difference that I'm aware of is that locally I used Julia v1.2 to build the doc instead of Julia v1.0.

This Gadfly documentation build with Julia 1.2 got through in this Travis job.

[CiaranOMara]

Commit a283bec got through in ~18min with travis_wait.