[patch] welcoming Mono in ir



In the progress of investigating why with the last git revision,
ir.exe in repl mode doesn’t print the result of the evaluation, I
figured I’d write the most useful patch of the year, to get a cute
prompt on Mono:

[mono] ~/Desktop/Debug @ mono ir.exe
IronRuby on Mono 2.5
Copyright © Microsoft Corporation. All rights reserved.

That’s the documented way to do so, and will be maintained that way.