Fix mismatched type exception thrown by mccabe.py.

This bug is a variation of the same one reported in issue #447.

Commit 00c6c75b on 2014-06-16 introduced this statement in plugin/pymode.vim:

    call pymode#default("g:pymode_lint_options_mccabe",
        \ {'complexity': 12})

That results in pymode/libs/pylama/lint/pylama_mccabe/mccabe.py throwing this
exception:

    Run mccabe {'complexity': '12'}
    <traceback object at 0x101eb9688>
    Traceback (most recent call last):
    File "/Users/krader/Dropbox/dotfiles/vim/bundle/python-mode/pymode/libs/pylama/core.py",
    line 68, in run
        select=params.get("select", set()), params=lparams):
    File "/Users/krader/Dropbox/dotfiles/vim/bundle/python-mode/pymode/libs/pylama/lint/pylama_mccabe/__init__.py", line 20, in run
        return get_code_complexity(code, complexity, filename=path) or []
    File "/Users/krader/Dropbox/dotfiles/vim/bundle/python-mode/pymode/libs/pylama/lint/pylama_mccabe/mccabe.py", line 267, in get_code_complexity
        for lineno, offset, text, check in McCabeChecker(tree, filename).run():
    File "/Users/krader/Dropbox/dotfiles/vim/bundle/python-mode/pymode/libs/pylama/lint/pylama_mccabe/mccabe.py", line 247, in run
        if self.max_complexity < 0:
    TypeError: unorderable types: str() < int()