Removing CommonCrypto to fix builds in Xcode 10 beta

[jondwillis]

See: #102

[kylef]

@jondwillis Can you rework this PR so it works on the current version of Xcode too?

[jonblatho]

I have a fix in #112 for this issue that works across both Swift 4.1.x and Swift 4.2. Earlier versions of Swift aren’t supported because #if canImport(ModuleName) was added in Swift 4.1.

I tried some other ways to get it working on earlier Swift versions and could not get them working reliably, but a better way may exist and I frankly didn’t invest too much time into it. In any case, feel free to borrow the fix if needed.

[Coeur]

@kylef According to an Apple news from September 2018, this month of March 2019 will see Xcode 10 being mandatory for publishing iOS apps. And Xcode 10 supports Swift 4.x and #if canImport(), so there is really no need to keep support for older versions of Xcode.