Use BinaryBuilder-provided binaries, switch CI to GHA

[ararslan]

This PR has three distinct commits:

• Switch the old BinaryProvider- and local system-based method of providing binaries to BinaryBuilder with Artifacts. Closes Use BinaryBuilder #36.
• Switch CI from Travis + AppVeyor to Github Actions. Travis and AV will likely need their status checks disabled manually.
• Bump the patch version to 0.3.4 for a new release.

[ararslan]

Ah, right, JLLs require Julia 1.3.

[nalimilan]

Thanks! Can you update README.md to say that GNU libiconv is used instead of saying that multiple implementations may be used? Also some docstrings could be updated.

[ararslan]

Done! Also, it looks like AV is running on your account, Milan, so you'll have to disable it.