fourflusher not honouring selected xcode

I'm aware there are a few issues in this repo already regarding disparity between current xcode version and cocoapods/fourflusher version, however I feel only the symptoms are being addressed and not the issue.

I could be wrong (so please correct me if I am), but it seems fourflusher isn't honouring the selected xcode version on the machine.

For example, we have a few build machines that have Xcode 10.1 and 10.2 installed alongside CocoaPods 1.5.3. With Xcode 10.1 selected, when I run `xcrun simctl list` I see a list of 12.1 simulators. Here is a snippet of the output:

```
== Runtimes ==
iOS 12.1 (12.1 - 16B91) - com.apple.CoreSimulator.SimRuntime.iOS-12-1
tvOS 12.1 (12.1 - 16J602) - com.apple.CoreSimulator.SimRuntime.tvOS-12-1
watchOS 5.1 (5.1 - 16R591) - com.apple.CoreSimulator.SimRuntime.watchOS-5-1
== Devices ==
-- iOS 12.1 --
iPhone 5s (62CAB7B3-76E1-4C65-8886-51ED36CA87D9) (Shutdown)
iPhone 6 (0AADE35F-36AA-40C2-B4C9-A67E576A2C2C) (Shutdown)
iPhone 6 Plus (3BBA817A-0CD7-4B84-BBD7-7B933315C5C2) (Shutdown)
...
```

However, when I run `pod spec lint` it says it can't find simulators that match the output (e.g. `com.apple.coresimulator.simruntime.ios-12-1`)

Fourflusher should use the selected / active version of Xcode (based on the output from `xcode-select -p` or `xcversion selected`).

Start with the entry points used by `pod spec lint` and compare how simulator discovery uses `xcode-select -p` or `xcversion selected` against `xcrun simctl list`. Reproduce the mismatch with the installed Xcode versions; done means the selected Xcode's simulator runtimes are recognized consistently.

Written by the indexing model from the issue text.