lib/lingo/attendee/vector_filter.rb: Added support for TFIDF scoring.
Rakefile (test_ref): Use Diff::LCS directly instead of going through Diff::LCS::Ldiff.run.
Rakefile (test_ref): Work around test failures on JRuby due to varying floating point precision.

https://travis-ci.org/blackwinter/lingo/jobs/43734013
Rakefile: Updated for hen 0.8.1.
.travis.yml: Use new container-based stack.