[travis] test on new XCode/macOS version with Java 8, 10, 11, 12 (#74)

https://docs.travis-ci.com/user/reference/osx/#macos-version

Author: tofi86

.travis.yml

@@ -1,18 +1,21 @@
 language: generic
 
 # run on different Mac OS X versions
-# https://docs.travis-ci.com/user/reference/osx/#OS-X-Version
+# https://docs.travis-ci.com/user/reference/osx/#macos-version
 matrix:
   include:
-    # OS X 10.11
+    # OS X 10.12, Java 8
     - os: osx
-      osx_image: xcode8
-    # OS X 10.12
+      osx_image: xcode9.2
+    # OS X 10.13, Java 10
     - os: osx
-      osx_image: xcode9
-    # OS X 10.13
+      osx_image: xcode10
+    # OS X 10.13, Java 11
     - os: osx
-      osx_image: xcode9.3
+      osx_image: xcode10.1
+    # OS X 10.14, Java 12
+    - os: osx
+      osx_image: xcode11
 
 
 # Homebrew needs ruby 2.3 and should be updated everytime to use the latest