Update apkdiff.py

[soulee-dev]

Problem

As mentioned in Issue #4 (comment)

Some of OS has problem with line

apkdiff/apkdiff.py

Line 81 in 2468350

call(["apktool", "d", "--no-debug-info", "-o", dir, file], stdout=open(os.devnull, 'w'), stderr=STDOUT)

Suggest

 call(["apktool", "d", "--no-debug-info", "-o", dir, file], stdout=open(os.devnull, 'w'), stderr=STDOUT) 

Shell should be true for OS compatibility.
If the shell is true, command will be executed through the shell according to Python Docs

[Sparrrgh]

This change broke the tool both in Windows and Ubuntu. Without shell=True it works fine