updates for ruby 2.4.2 and rails 5.1.4

[jamesvanmil]

closes #325

• .ruby-version bumped to 2.4.2
• rails bumped to 5.1.4
• unpinned all gems in Gemfile
• migrated migrations - you're now required to specify which version of rails the migration is for

Notes:

• Tests are slower I think - I think they're actually talking to the postgresql server now - required to be in the network for them to pass
• I haven't been able to test this with real data
• because all gems are unpinned, we'll need to pay more attention when doing a bundle update vs a bundle install

[jamesvanmil]

Something has changes with stubbing, and simply stubbing BibView isn't working to checking the database for a table. So this marks as pending a test that works from our network, but not on Travis.

[billmcmillin]

Working on resolving error thrown by empty strings passed to to_d with new ruby. See https://jamescrisp.org/2017/09/26/bigdecimal-fix-for-rails-4-with-ruby-2-4/