Update .travis.yml

[BookOfGreg]

• Remove Ruby 1.9
• Add Ruby 2.4
• Update JRuby to 9.1.9.0 which builds on react-rails' travis
• Add Fast Finish to matrix (Returns status as soon as anything fails or just before allowed fails)
• Attempt to cache Bundler due to comment on L36-37

(attempt to) Fixes #221

TODO:

• [x] PhantomJS currently fails to download.

[BookOfGreg]

Well good to see Fast Finish on fail works! 😆

[BookOfGreg]

For maintainers: Tests for Opal 0.8 appear to fail in the current build but this appears unrelated to the travis config.