Fix binary wheel build step

[moralrecordings]

PyPI expects Linux binaries to be built to the manylinux specification. The build toolchain for V8 uses a new-ish glibc, so this process produces manylinux2014-compatible packages.

I've tested the cibuildwheel build process, but the changes to the Travis build script are speculative based on the manual process I used. With luck opening a PR will fire off a proper test.

[tbodt]

Please mark this as ready for review when it's done

[scottp-dpaw]

Notes:

• I gave up on cibuildwheel, as it was a bit flaky, and there isn't an official Docker image for manylinux2014 with Python 2.7 support. The v8 toolchain is manylinux2014 symbol compatible, so we build everything with that and then confirm the result with auditwheel.
• For some reason, the build scripts for the v8 unit testing framework generate Clang commands that exclude the compiler's default paths for includes etc, leading to the build failing. Given that the tests are unused by v8py, and there is no obvious way to exclude it, I made a change to setup.py to strip out the test targets from v8.
• greenstack isn't compatible with Python 3.7 or 3.8, so the Travis job checks the version before attempting to install.

[tbodt]

Thanks for the PR. It'll be good to finally have wheels, 3 years later. There are a few areas of future work I've noted, otherwise looks good to me (as long as it actually works).

If you don't mind me asking, what are you using v8py for?

[armudgal]

This is great. Thanks for the PR