Xcode 10 beta & Swift 4.2 support

[MaddyCoder]

• I've read, understood, and done my best to follow the *CONTRIBUTING guidelines.

What did you do?

ℹ Tried running my app which has charts library integrated on Xcode 10.

What did you expect to happen?

ℹ I expected charts library to run on Xcode10.

What happened instead?

ℹ Got Compiler error.

Charts Environment

Charts version/Branch/Commit Number:
**Xcode version:**Xcode 10 beta
Swift version: 4.2
Platform(s) running Charts: iPad
macOS version running Xcode:

[misuqian]

same

[liuxuan30]

Use the PR #3522 for now. We will update once Xcode 10 is released.