(MAINT) Pin Boxstarter to V 2.9.26

Boxstarter is misbehaving again, so pin to earlier version.
Issue chocolatey/boxstarter#265 logged
against Boxstarter.

scripts/windows/start-boxstarter.ps1

@@ -63,8 +63,22 @@ if ($WindowsVersion -like $WindowsServer2008 ) {
   }
 }
 else {
-  iex ((new-object net.webclient).DownloadString('https://raw.githubusercontent.com/mwrock/boxstarter/master/BuildScripts/bootstrapper.ps1'))
-  Get-Boxstarter -Force
+  # Latest version of Boxstarter is breaking the build, so pinning to Version 2.9.26 until it is resolved.
+  # Issue logged https://github.com/mwrock/boxstarter/issues/265
+
+  $BoxstarterPackages = @(
+    'boxstarter',
+    'boxstarter.common',
+    'boxstarter.winconfig',
+    'boxstarter.bootstrapper',
+    'boxstarter.chocolatey',
+    'boxstarter.hyperv'
+  )
+  $BoxstarterPackages | % {
+    iex ((New-Object System.Net.WebClient).DownloadString('https://chocolatey.org/install.ps1'))
+    choco install $_ -y --force --version "2.9.26" --argsglobal=true --paramsglobal=true --ignoredependencies=true
+    choco pin add --name="$_"
+  }
 }
 
 # Cleanup after boxstarter install