Fix failing Travis builds:

Work around an unresolved Travis/Bundler/RubyGems bug:

* travis-ci/travis-ci#8978
* travis-ci/travis-ci#8969

Remove `dist: trusty` as that's the default:

* https://blog.travis-ci.com/2017-07-11-trusty-as-default-linux-is-coming
* https://blog.travis-ci.com/2017-08-31-trusty-as-default-status

Use the recommended Rubinius version (rbx-3):

* travis-ci/docs-travis-ci-com#1534

Cache dependencies to speed up builds:

* https://docs.travis-ci.com/user/caching/

Automatically choose the latest versions of 2.x (e.g. currently 2.4.4 for 2.4):

* rvm/rvm#4352 (comment)

.gitignore

@@ -2,3 +2,4 @@ pkg/
 *.rbc
 .bundle/
 doc/
+/vendor/

.travis.yml

@@ -1,12 +1,15 @@
 sudo: false
 language: ruby
-dist: trusty
+cache: bundler
 rvm:
-  - 2.4.0
-  - 2.5.1
-  - jrub
-  - rbx
+  - 2.4
+  - 2.5
+  - jruby
+  - rbx-3
   - ruby-head
+before_install:
+  - gem update --system
+  - gem install bundler
 matrix:
   allow_failures:
-      - rvm: rbx
+      - rvm: rbx-3