What's the cmd/PowerShell equivalent of back tick on Bash?

In CMD.EXE there is no direct equivalent. But you can use the FOR command to achieve what you want.

Do something like the following:

FOR /F "usebackq" %x IN (`./print_5_As.rb`) DO @echo Foo %x

or

FOR /F %x IN ('"./print_5_As.rb"') DO @echo Foo %x

You might need to set delimiter to something else than the default, depending on how the output looks and how you want to use it. More details available in the FOR documentation at https://technet.microsoft.com/en-us/library/bb490909.aspx