Locally install swiftlint

[jsorge]

When I first cloned the repo I got a build failure because I didn't have swiftlint installed using Homebrew. Instead of having that be the preferred behavior, this PR adds a script to locally download swiftlint to the repo and runs the script using that version.

To change the version of swiftlint the project uses, update the .swiftlint-version file at the source root.

Pros: • No more build failures because of a tool needing to be installed outside of the repo • Everyone will be using the same version of swiftlint

Cons: • A couple of additional seconds on the first build before swiftlint is downloaded.

[twostraws]

It's odd that you got a build error – I was under the impression that the build script only runs SwiftLint if it finds it.

[veb]

I don't have it installed, just checked homebrew too:

And it builds perfectly fine for me.

[jsorge]

Oh weird, it actually turns out that my brew'd swiftlint installation was hosed and would fail every time. When I removed it the project builds just fine without it.

So, we can close this out without merging or merge it if we want folks to consistently lint the project.

[davedelong]

I'm not sure how I feel about this. I generally don't approve of installing software on peoples' machines without their direct consent. The redeeming thing here is that it's installed locally to the git clone.

On the other hand, this does not account for SwiftLint already existing on their machine.