Fix test_getpcmd, which occasionally fails

This have been pestering me and other luigi developers a lot. For
example, when you send a correct PR, it will sometimes fail on Travis
and you have to retrigger the bulid.

For example, it happened to me recently. This is what Travis prints:

    ======================================================================
    FAIL: test_getpcmd (lock_test.TestCmd)
    ----------------------------------------------------------------------
    Traceback (most recent call last):
      File "/home/travis/build/spotify/luigi/test/lock_test.py", line 32, in test_getpcmd
        "sleep 1"
    AssertionError: '[sleep]' != 'sleep 1'

    ----------------------------------------------------------------------
Merge pull request #529 from Tarrasch/fix_getpcmd_test

Fix test_getpcmd, which occasionally fails