Bug in triggered build travis matrix

[springmeyer]

It looks like triggered builds are not creating the matrix correctly because we are getting 4 jobs rather than 2. For example for gdal 2.0.2 this matrix is used with 2 entries (one for each operating system) but travis gets 4: https://travis-ci.org/mapbox/mason/builds/121252415

/cc @jfirebaugh in case he has ideas.

[jfirebaugh]

I believe this is a Travis bug: travis-ci/travis-ci#4681

[springmeyer]

Thanks @jfirebaugh - I was aware of that behavior, but confused because we are not using any global setting like compiler. But then I realized that the main travis.yml was likely being inherited and we have a global os setting. Removing that fixed the problem: 5026db6.

I've been thinking we should ditch testing on osx anyway to avoid the delays. If we want to re-introduce OS X testing for the main repo run we could likely dodge this bug by nesting the os inside matrix: include.