Run all C Extension builds on a single builder

[natefoo]

We run separate Linux builders for each Python version mainly for parallelism (although this repo is currently limited to 1 concurrent builder so as not to dominate the org's Travis allotment), but it'd be entirely possible to run all of the Linux builds from a single builder. In fact, the division is somewhat arbitrary, since the Linux builders build Docker images and then build both 32- and 64-bit wheels in a single builder (and in the case of Python 2.7, a total of 4 wheels due to cp27m and cp27mu ABIs).

Likewise for macOS, we could do all versions in a single builder, although in the case of macOS there'd be less isolation between builds since there's no extra level of virtualization in those builders like there is in Linux, and you'd be installing all 5 CPython interpreters in the builder at once.

Pros:

• Less complexity in Travis/Starforge interaction especially for C Extension wheels that don't build on the full default-wheels imageset (see Short-circuit builders for C Extension wheels not using the default imageset #11)
• Little difference in build time on the main repo due to build limit

Cons:

• Slows down testing on your own fork (where you have greater Travis parallelism)
• Harder to see if a wheel fails on a given Python version - it'll fail the job but you have to scroll through the output to determine exactly what images failed (but Starforge can be updated to output a build summary at the end to alleviate this somewhat)