Include platyPS in required tools

[rjmholt]

Just tried to build PSES on a new machine on Windows and got:

Done /./CITest/?Test/TestProtocol 00:00:08.4099755
Done /./CITest/?Test 00:00:27.1330013
Task /./CITest
Done /./CITest 00:00:27.1450300
Task /./BuildCmdletHelp
ERROR: The term 'New-ExternalHelp' is not recognized as the name of a cmdlet, function, script file, or operable program.
Check the spelling of the name, or if a path was included, verify that the path is correct and try again.
At C:\Users\rob\Documents\Dev\Microsoft\PowerShellEditorServices\PowerShellEditorServices.build.ps1:316 char:5
+     New-ExternalHelp -Path $PSScriptRoot\module\docs -OutputPath $PSS ...
+     ~~~~~~~~~~~~~~~~
At C:\Users\rob\Documents\Dev\Microsoft\PowerShellEditorServices\PowerShellEditorServices.build.ps1:315 char:1
+ task BuildCmdletHelp {
+ ~~~~~~~~~~~~~~~~~~~~~~
At C:\Users\rob\Documents\Dev\Microsoft\PowerShellEditorServices\PowerShellEditorServices.build.ps1:343 char:1
+ task . GetProductVersion, Clean, Build, TestPowerShellApi, CITest, Bu ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Build FAILED. 13 tasks, 1 errors, 0 warnings 00:00:59.5884881
New-ExternalHelp : The term 'New-ExternalHelp' is not recognized as the name of a cmdlet, function, script file, or operable program.
Check the spelling of the name, or if a path was included, verify that the path is correct and try again.
At C:\Users\rob\Documents\Dev\Microsoft\PowerShellEditorServices\PowerShellEditorServices.build.ps1:316 char:5
+     New-ExternalHelp -Path $PSScriptRoot\module\docs -OutputPath $PSS ...
+     ~~~~~~~~~~~~~~~~
+ CategoryInfo          : ObjectNotFound: (New-ExternalHelp:String) [], ParentContainsErrorRecordException
+ FullyQualifiedErrorId : CommandNotFoundException

Looks like we need to include platyPS in our list of dependencies that we check.