many-to-many association between snps and papers
fixing tests

also stubbing Solr and all Sidekiq enqueue calls
fixing tests
restoring .travis.yml
don't build debugger gem on Travis
adding migration for references
stubbing Sidekiq enqueue
renaming Reference to SnpReference

Turns out, 'reference' is a reserved word in PostgreSQL.
Merge remote-tracking branch 'origin/master' into fix/many_to_many_snp_papers

Conflicts:
	Gemfile
	app/models/snp.rb
	test/factories.rb
	test/test_helper.rb
Pluralizing pgp_annotations association in Snp
Adding test for Snpedia worker
Refactoring Snpedia worker
allow_http_connections_when_no_cassette in the VCR
Adding test for GenomeGov worker
Refactoring GenomeGov worker
Improving logging in PlosSearch worker
Don't break when a PLoS article has no authors

fixes https://www.exceptional.io/exceptions/560531842
Removing commented lines
Removing snp_id from PlosSearch worker
Fixing GenomeGov worker