add future to requirements.txt

Python 3 support requires using builtins in commit https://github.com/open-data/ckanext-xloader/commit/8c43ceed879af9934dd843a36ab7c68d9a0535c8#diff-b0f564951458e386ad6106e20fa8938150638c260b1921aceba34dae89d04d58. Adding future to the requirements to avoid ImportError.