.travis.yml: Use rvm gemsets (fixes bundler issue)

Makes the default bundler v2.x.x unavailable,
so that the older bundler v1.17.3 can be used instead.

.travis.yml

@@ -1,12 +1,13 @@
 language: ruby
 dist: xenial
 before_install:
-  - gem install rubygems-update && update_rubygems
   # Rails 4.2 doesn't support bundler 2.0, so we need to lock bundler to
   # v1.17.3. This is just for Ruby 2.5 which ships with bundler 2.x on Travis
   # CI while Ruby 2.6 does not.
   # https://github.com/travis-ci/travis-rubies/issues/57#issuecomment-458981237
-  - yes | rvm @global do gem install bundler -v 1.17.3 || true
+  - rvm gemset create travis
+  - rvm gemset use travis
+  - gem install bundler -v 1.17.3
 rvm:
   - 2.3.8
   - 2.4.6