App fails with system python before OSX system check

Got the following error when running python instead of python2.7.

This pull request moves up the existing version check so we get the nice error message.

    master-blaster:PyBitmessage grant$ python src/bitmessagemain.py 
    Not using the gevent module as it was not found. No need to worry.
    Loading existing config files from /Users/grant/Library/Application Support/PyBitmessage/
    Traceback (most recent call last):
      File "src/bitmessagemain.py", line 27, in <module>
        from class_sqlThread import *
      File "/Users/grant/src/PyBitmessage/src/class_sqlThread.py", line 2, in <module>
        import shared
      File "/Users/grant/src/PyBitmessage/src/shared.py", line 398, in <module>
        from debug import logger
      File "/Users/grant/src/PyBitmessage/src/debug.py", line 71, in <module>
        configureLogging()   
      File "/Users/grant/src/PyBitmessage/src/debug.py", line 27, in configureLogging
        logging.config.dictConfig({
    AttributeError: 'module' object has no attribute 'dictConfig'