travis-ci: update list of JDKs in CI

Added java-8 (LTS) and java-12 (most recent) JDKs, removed openjdk10
(EOL) and oraclejdk11.

The idea is to test against widely used JDKs (LTS ones) and also against
a most recent JDK (to catch possible problems with new versions of java
early).

OracleJDK is built from OpenJDK starting from java-7, so there is no
much need to test against it.

Author: isopov

.travis.yml

@@ -15,11 +15,11 @@ jobs:
       script: mvn checkstyle:check
       after_success: skip
     - stage: test
-      jdk: oraclejdk11
-    - stage: test
-      jdk: openjdk10
+      jdk: openjdk8
     - stage: test
       jdk: openjdk11
+    - stage: test
+      jdk: openjdk12
 
 before_script:
   - .travis/travis.pre.sh