Fix YARD warnings

MSP-11068
Exit 1 if undocumented objects in yard:stats

MSP-11068
Allow yard:stats threshold to be adjusted

MSP-11068

config/yard-stats-threshold can be used to set the percentage documented
required for yard:stats to pass.
Full documentation

MSP-11068

Remove `config/yard-stats-threshold` and document all undocumented
objects.
Document how to use yard:stats in CI

MSP-11068
Fix typo in .travis.yml

MSP-11068

RAKE_SPEC -> RAKE_TASK
Run `rake yard` on travis-ci to catch undocumented objects

MSP-11068
Update cucumber features for `rake yard:stats` thresholding

MSP-11068