Xcode 10.1 (10B61) missing support

* [x] I've read, understood, and done my best to follow the [*CONTRIBUTING guidelines](https://github.com/jjatie/Charts/blob/master/CONTRIBUTING.md).

## What did you do?

ℹ Install the current AppStore Version of Xcode (version 10.1, Build number 10B61).
Try building the `Charts` framework target.

## What did you expect to happen?

ℹ Build is successfully working and creates a build result.

## What happened instead?

ℹ The framework is currently not building and results in_

```
:0: error: Swift does not support the SDK 'iPhoneOS11.2.sdk'
Command CompileSwiftSources failed with a nonzero exit code
```

Full [build log](https://github.com/danielgindi/Charts/files/2533602/build-log.txt) attached.

## Charts Environment

**Charts version/Branch/Commit Number: 3.2.1**
**Xcode version: version 10.1, Build number 10B61**
**Swift version: 4.2**
**Platform(s) running Charts: iOS**
**macOS version running Xcode: macOS Mojave 10.14 (18A391)**

## Demo Project

ℹ https://github.com/danielgindi/Charts

Start by reproducing the Charts framework target build with Xcode 10.1 and review the linked build log, focusing on the reported iPhoneOS11.2 SDK error. Compare the project’s build configuration with the Swift 4.2 and iOS environment described in the issue; done means the framework builds successfully and produces a build result.

Written by the indexing model from the issue text.