Update rebar_utils.erl

On windows, bootstrap.bat failed with next error.
Command 'escriptize' not understood or not applicable

This happens because the drive name in path got from rebar_utils:get_cwd() and base_dir(Config) are different case.
Made the drive name the same lowercase using filename:absname().
Do not wrap base_dir with filename:absname() 

base_dir() returns already filename:absname()'ed path.
Merge pull request #274 from redpine50/broken_on_win

Use lowercase for Windows drive name to resolve issue #250