Fix WTForms version requirement#51

WTForms-JSON 0.2.8 should have updated the wtforms requirement for version >= 1.0.5, but that was not done. Relevant commits: [wtforms-json 0.2.8](https://github.com/kvesteri/wtforms-json/commit/02b5b43bfc4bb0d046e5d9f90df2dea29c155a2d) depends on [wtforms 1.0.5](https://github.com/wtforms/wtforms/commit/04bc9c137c021fc14e3382400ba1e7ab93498a96). The tests are also using wtforms 1.0.5, which is why this mistake was not found. Trying to use wtforms < 1.0.5 will cause the following error when `wtforms_json.init()` is called: ``` File "/home/ubuntu/virtualenvs/venv-2.7.11/lib/python2.7/site-packages/wtforms_json/__init__.py", line 282, in init BooleanField.false_values = BooleanField.false_values + (False,) AttributeError: type object 'BooleanField' has no attribute 'false_values' ```