Fixed to work with Linux kernel 3.x.

os.platform returns 'linux3' if you build your python with linux kernel 3.x.
Since Python 3.3, it returns 'linux' so it should be used with startswith('linux').

cf. http://docs.python.org/dev/library/sys.html#sys.platform
Merge pull request #964 from matsuu/fix-linux-3

Fixed to work with Linux kernel 3.x.