fix issue for relative symlinked brew command

New brew command is not worked when it is relative symlinked.

    imac:~% brew
    /Users/don/local/bin/brew: line 11: /Users/don/../homebrew/bin/../Library/brew.rb: No such file or directory
    /Users/don/local/bin/brew: line 11: exec: /Users/don/../homebrew/bin/../Library/brew.rb: cannot execute: No such file or directory

I solved this problem by converting relative path to absolute path.