Switch to using joblib MemmappingPool to fix #112

Also removes requirement for concurrent.futures, since we're not using
it anymore.
Remove extra lines from data.py and up joblib requirement to 0.8
Add missing pyyaml installation to .travis.yml
Add call to pool.terminate
Merge pull request #142 from EducationalTestingService/feature/add_memmapping_pool

Switch to using joblib's MemmappingPool for loading files