Update Travis CI Ruby versions.

Add RubyGems and Bundler version restrictions from master.

.travis.yml

@@ -2,7 +2,10 @@ language: ruby
 dist: trusty
 sudo: false
 before_install:
-  - gem update bundler
+  - if [[ $TRAVIS_RUBY_VERSION =~ ^(1|2\.[012]|jruby-1)\. ]]; then gem install rubygems-update --version '~> 2.7' --no-document && update_rubygems; else gem update --system; fi
+  - gem --version
+  - if [[ $TRAVIS_RUBY_VERSION =~ ^(1|2\.[012]|jruby-1)\. ]]; then gem install bundler --version '~> 1.17'; else gem install bundler; fi
+  - bundle --version
 before_script:
   - bundle update
 cache: bundler
@@ -13,14 +16,16 @@ rvm:
   - 2.0.0-p648
   - 2.1.10
   - 2.2.10
-  - 2.3.7
-  - 2.4.4
-  - 2.5.1
+  - 2.3.8
+  - 2.4.9
+  - 2.5.7
+  - 2.6.5
+  - 2.7.0-preview3
   - ruby-head
   - jruby-18mode
   - jruby-1.7.27
   - jruby-9.1.17.0
-  - jruby-9.2.0.0
+  - jruby-9.2.9.0
   - jruby-head
   - rbx-2.71828182
   - rbx-3.107