Linux support

[heckj]

resolves #11

[heckj]

I couldn't vet the linux CI without some tweaking, so I saved that for last, will temp enable it on branches, but I vetted the code with:

swift test # checkin' mac local
brew install iainsmith/formulae/swift-docker # handy tool
swift docker test --args --enable-test-discovery # checkin' linux w/ swift 5.2

[kean]

Let me wrangle with CI later, I'm having some issues with GitHub integration.

Are you planning to do any additional changes or is it ready to be merged?

[heckj]

CI aside, that should be ready to roll - and I'm happy to rebase and squash this all if that would be easier for you.

[kean]

That's fine, let me do some testing later today before I merge it. Thanks for putting this together.

[kean]

@heckj, I updated the .travis.yml file. You can't use matrix in the kind of setup that I have, so I changed it a bit. I see that the tests on Linux have finished successfully https://travis-ci.org/github/kean/Future/builds/680205461 👍

[heckj]

That's fantastic! I'm excited to see that running in CI for this project, and with that - I'm going to turn around and take a stab at using your awesome work within another project (ref SwiftDocOrg/DocTest#1) where it needs some synchronization support while running on Linux.