CI: Update travis Ruby versions

[Aesthetikx]

Description

Towards a release supporting 2.7.x, this changes updates the travis builds:

• Ruby 2.3.x is EOL as of 2019-03-31,
• Ruby 2.7.0 is out as of 2019-12-15.

The other ruby minor versions seem up to date afaict.

Testing

Waiting for a green traivs build.

References

• Ruby Maintenance Branches

[Aesthetikx]

Looks like the remaining deprecation warnings may be from the mongoid dependency, but it is hard to tell with those stack traces.