(GH-48) Set TLS 1.2 if possible

shell/InstallChocolatey.ps1

@@ -11,6 +11,20 @@ $env:Path += ";$ChocoInstallPath"
 $DebugPreference = "Continue";
 $env:ChocolateyEnvironmentDebug = 'true'
 
+# PowerShell will not set this by default (until maybe .NET 4.6.x). This
+# will typically produce a message for PowerShell v2 (just an info
+# message though)
+try {
+    # Set TLS 1.2 (3072) as that is the minimum required by Chocolatey.org.
+    # Use integers because the enumeration value for TLS 1.2 won't exist
+    # in .NET 4.0, even though they are addressable if .NET 4.5+ is
+    # installed (.NET 4.5 is an in-place upgrade).
+    [System.Net.ServicePointManager]::SecurityProtocol = [System.Net.ServicePointManager]::SecurityProtocol -bor 3072
+}
+catch {
+    Write-Output 'Unable to set PowerShell to use TLS 1.2. This is required for contacting Chocolatey as of 03 FEB 2020. https://chocolatey.org/blog/remove-support-for-old-tls-versions. If you see underlying connection closed or trust errors, you may need to do one or more of the following: (1) upgrade to .NET Framework 4.5+ and PowerShell v3+, (2) Call [System.Net.ServicePointManager]::SecurityProtocol = 3072; in PowerShell prior to attempting installation, (3) specify internal Chocolatey package location (set $env:chocolateyDownloadUrl prior to install or host the package internally), (4) use the Download + PowerShell method of install. See https://chocolatey.org/docs/installation for all install options.'
+}
+
 function Install-LocalChocolateyPackage {
 param (
   [string]$chocolateyPackageFilePath = ''