Switch build to Java 11

[badgerwithagun]

Fixes #113.

This PR will be kept working and up-to-date and merged when it's the right time to switch Orko entirely.

[badgerwithagun]

Many of these changes are gradually getting merged into master as version bumps to dependencies. Transition route:

• Gradually reduce this down, via rebasing, to just the compiler and version changes
• Enable CI for Java 11 (including E2E tests)
• Stabilise for a while and wait for Java 11 to achieve stable state
• Switch over

[badgerwithagun]

Needs manual inclusion of the javax.activation and javax.bind libraries. See:

https://github.com/dropwizard/dropwizard/issues/2318

[badgerwithagun]

Should resolve this warning

[badgerwithagun]

Once #399 is in, it should be relatively easy to merge the code changes for this in and create a parallel Java 11 build on CircleCI so Java 11 support is maintained as part of the normal development train.

[badgerwithagun]

I think the final trigger for this migration will be when dropwizard 2.0.0 drops. That will finally remove all the hacks necessary to get DW working on Java 11. I'm working on bringing the bulk of these changes into master now, barring the java 11 compiler requirement in the POM and CircleCI.