Update bootstrap.ps1 for PowerShell Core on Windows

[matt9ucci]

When running bootstrap.ps1 on PowerShell Core on Windows, the following message is displayed:

Start-DotnetBootstrap cannot be run in Core PowerShell on Windows (need Invoke-WebRequest!)

But bootstrap.ps1 can be runnable on PowerShell Core on Windows because it has Invoke-WebRequest as a built-in cmdlet.

I have modified the if statement and tested it on the following environment:

PS> $PSVersionTable

Name                           Value
----                           -----
PSVersion                      6.1.0
PSEdition                      Core
GitCommitId                    6.1.0
OS                             Microsoft Windows 10.0.17134
Platform                       Win32NT
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0...}
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1
WSManStackVersion              3.0