resolve unnecessary download issue #137 

Found some issues in the way version compare was being performed.

Issue 1. _find_latest_package() included the "key" in the resulting versions list.
    ver = "build-tools-20.0.0"
    v_build_tools = "20.0.0"
    ver > v_build_tools
    --> True

Issue 2. String comparison is being performed.
    It appears to have worked ok so far, but strings do not evaulate like numbers.

    v4_2_2 = "4.2.2"
    v4_2_16 = "4.2.16"
    v4_2_2 > v4_2_16
    --> True

    Updated code to parse version_string to list of ints, for example "4.2.2" --> [4,2,2]
    Note this implementation assumes versions are the same length.