.travis.yml in rmagick-3.0.0 vs .travis.yml in rmagick-3.1.0

@@ -1,27 +1,40 @@
 language: ruby
 
 cache:
   directories:
     - ~/.ccache
+    - vendor/bundle
+    - build-ImageMagick
 
 os:
   - linux
 
 sudo: required
 
 env:
+  - IMAGEMAGICK_VERSION=6.7.7-10
+  - IMAGEMAGICK_VERSION=6.7.7-10 CONFIGURE_OPTIONS=--enable-hdri
   - IMAGEMAGICK_VERSION=6.8.9-10
   - IMAGEMAGICK_VERSION=6.8.9-10 CONFIGURE_OPTIONS=--enable-hdri
+  - IMAGEMAGICK_VERSION=6.9.10-40
+  - IMAGEMAGICK_VERSION=6.9.10-40 CONFIGURE_OPTIONS=--enable-hdri
 
 before_install:
   - source before_install_$TRAVIS_OS_NAME.sh
 
-install: bundle install --verbose
+install: bundle install --path=vendor/bundle --verbose
 
 rvm:
   - 2.3
   - 2.4
+  - 2.5
+  - 2.6
+
+matrix:
+  include:
+    - rvm: 2.3
+      env: STYLE_CHECKS=true IMAGEMAGICK_VERSION=6.8.9-10
 
 notifications:
   webhooks:
     urls:
       - https://webhooks.gitter.im/e/a98c1112f5d60f97a532