check if blocking project exist before changing to lower

I got following expectation while trying to use it. This patch fixes it.

caniusepython3 -r requirements/base.txt
Finding and checking dependencies ...
WARNING:root:Stale overrides: set([u'pyopenssl'])
Traceback (most recent call last):
  File "/Users/neo/.virtualenvs/ss/bin/caniusepython3", line 8, in <module>
    load_entry_point('caniusepython3==1.0', 'console_scripts', 'caniusepython3')()
  File "/Users/neo/.virtualenvs/ss/lib/python2.7/site-packages/caniusepython3/__main__.py", line 119, in main
    blockers = ciu.blocking_dependencies(projects, ciu.all_py3_projects())
  File "/Users/neo/.virtualenvs/ss/lib/python2.7/site-packages/caniusepython3/__init__.py", line 169, in blocking_dependencies
    for project in projects if project.lower() not in py3_projects]
AttributeError: 'NoneType' object has no attribute 'lower'