Windows install script: "Could not create SSL/TLS secure channel"

[StevenTammen]

The problematic commands and error messages they generate:

powershell -Command "Invoke-WebRequest https://github.com/yihui/tinytex/raw/master/tools/texlive.profile -OutFile texlive.profile"
Invoke-WebRequest : The request was aborted: Could not create SSL/TLS secure channel.
At line:1 char:1
+ Invoke-WebRequest https://github.com/yihui/tinytex/raw/master/tools/t ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (System.Net.HttpWebRequest:HttpWebRequest) [Invoke-WebRequest], WebExc
   eption
    + FullyQualifiedErrorId : WebCmdletWebResponseException,Microsoft.PowerShell.Commands.InvokeWebRequestCommand

powershell -Command "Invoke-WebRequest https://github.com/yihui/tinytex/raw/master/tools/pkgs-custom.txt -OutFile pkgs-custom.txt"
Invoke-WebRequest : The request was aborted: Could not create SSL/TLS secure channel.
At line:1 char:1
+ Invoke-WebRequest https://github.com/yihui/tinytex/raw/master/tools/p ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (System.Net.HttpWebRequest:HttpWebRequest) [Invoke-WebRequest], WebExc
   eption
    + FullyQualifiedErrorId : WebCmdletWebResponseException,Microsoft.PowerShell.Commands.InvokeWebRequestCommand

Powershell version, etc.

Running on Widows 10:

$PSVersionTable

Name                           Value
----                           -----
PSVersion                      5.1.16299.431
PSEdition                      Desktop
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0...}
BuildVersion                   10.0.16299.431
CLRVersion                     4.0.30319.42000
WSManStackVersion              3.0
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1

Other considerations

The generic fix for this issue appears to be forcing powershell to use TLS 1.2 instead of 1.0.
As in this StackOverflow thread.

Prefixing the Invoke-WebRequest lines with the code to change the TLS works, but it must be a temporary setting since it is not persistant across multiple calls of powershell -Command. I know virtually nothing about PowerShell, but maybe there is a way to set the setting globally?

I'm pretty new to Git, but have the issue fixed locally (though perhaps somewhat hackishly). I'll try to send a PR later today to fix this.