PowerShell bootstrap script not working with PS 6

@kentcb commented on [Tue Dec 06 2016](https://github.com/cake-build/cake/issues/1393)

### What You Are Seeing?

When running the [bootstrap script](https://github.com/cake-build/example/blob/master/build.ps1) on PowerShell 6 (.NET core):
```
Method invocation failed because [System.Reflection.Assembly] does not contain a method named 'LoadWithPartialName'.
At C:\XXX\build.ps1:58 char:1
+ [Reflection.Assembly]::LoadWithPartialName("System.Security") | Out-N ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : InvalidOperation: (:) [], RuntimeException
+ FullyQualifiedErrorId : MethodNotFound
```

`$PSVersionTable.PSVersion` gives me:

```
Major Minor Patch Label
----- ----- ----- -----
6 0 0 alpha
```

### What is Expected?

Script should work with new versions of PS.

### What version of Cake are you using?

Haven't gotten that far yet (bootstrapping).

### Are you running on a 32 or 64 bit system?

64 bit.

### What environment are you running on? Windows? Linux? Mac?

Windows 10, but the same behavior _should_ be present on other operating systems.

### Are you running on a CI Server? If so, which one?

No

### How Did You Get This To Happen? (Steps to Reproduce)

1. Grab the bootstrap script
2. Attempt to execute it in a PS 6 session

### Output Log

N/A