.travis.yml: Update to match other crypto-rb projects

.travis.yml

 language: ruby
-sudo: false
+cache: bundler
+
+before_install:
+  - gem update --system
+  - gem --version
+  - gem install bundler -v 1.16.1
+  - bundle --version
+
+bundler_args: --without development
 
 rvm:
-  - 2.2.8
-  - 2.3.5
-  - 2.4.2
-  - ruby-head
-  - jruby-9.1.14.0
-  - jruby-head
-  - rbx-2
+  - jruby-9.1.15.0
+  - 2.2
+  - 2.3
+  - 2.4.3
+  - 2.5.0
 
 matrix:
   fast_finish: true
-  allow_failures:
-    - rvm: ruby-head
-    - rvm: jruby-head
-    - rvm: rbx-2
 
-notifications:
-  irc: "irc.freenode.org#cryptosphere"
+branches:
+  only:
+    - master