(PUP-1937) Calling 'ruby' on Windows Fails for PE

The 'ruby' executable does not exist on a PE installation. Instead, the
real name is 'ruby.exe' and it is located within the PE installation
directory. This commit includes a new method (ruby_cmd) in WindowsUtil
for finding the correct executable name and fixes three tests that call
'ruby' directly on Windows.
Merge branch 'stable'