Fix Travis CI build errors. Closes #1224

[abevoelker]

Closes #1224. There were a few main causes:

• bundler was missing on jruby-19mode in Travis CI's image so I slightly modified a suggested fix from here: On Trusty, Rubies installed on demand lack bundler travis-ci/travis-ci#5578 (comment)
• the pg gem released a new 1.0.0 version yesterday which caused issues with Rails versions < 5 requiring ~> 0.11 at runtime, and Rails 5 requiring ~> 0.18 (figuring this out was slightly complicated by rake_test_loader swallowed a load error ruby/rake#247)
• the activerecord-jdbc-adapter gem released version 50.0 semi-recently, which should only be used by Rails 5+

Also, I don't know whether the intention is for the Gemfile in the root of the project to run agains the latest version of Rails or not, but mongoid being pinned to 2.6.0 is forcing it to select Rails 3, which is why I had to apply the version restriction on activerecord-jdbcpostgresql-adapter there to get the tests to pass.

[alexreisner]

Thanks for this! Really glad to have the tests fixed. Also you bring up a good point about Mongoid being pinned to 2.6.0. That was for Ruby 1.8 compatibility, which has since been dropped. So we don't need to pin it anymore. You want to fix that too?

[abevoelker]

Sure; I tested that change in a separate branch, however that surfaced one new build error which will require a new version of the activerecord-jdbcpostgresql-adapter to be published: jruby/activerecord-jdbc-adapter#780 (comment)

The build error could probably be worked around right now by changing the Gemfile to use the GitHub source instead:

gem 'activerecord-jdbcpostgresql-adapter', github: 'jruby/activerecord-jdbc-adapter'

But rather'n dirtying up the Gemfile it's probably preferable to just wait for the new gem version to be cut.

[alexreisner]

What about locking mongoid and activerecord-jdbcpostgresql-adapter so the most recent combo that works? I misspoke when I said we don't need to pin it at all--if we don't, the build constantly breaks (as illustrated). (Sorry!)

[abevoelker]

No preference here, it's your show 😄

Unfortunately I can't put more time on this PR; I'll have to bow out now as I've ran out of personal time. I leave it in your (or someone else's) capable hands to tidy up the Gemfile to your liking ✌️

[alexreisner]

Sounds good--I'll finish this up. Appreciate the help!