Actually check if the submodule is there

Actually checks for .git. See:

````
C:\Users\Tod Beardsley\Documents\GitHub\meterpreter>make
Meterpreter's submodule dependencies can't be found.
From your git console, please run:
  $ git submodule init && git submodule update

C:\Users\Tod Beardsley\Documents\GitHub\meterpreter>
````

Fixes rapid7#64