[ci] Move all jdks to matrix

Author: harawata

.travis.yml

@@ -1,15 +1,12 @@
 language: java
 
-jdk:
-  - openjdk15
-  - openjdk14
-  - openjdk11
-  - openjdk8
-
 matrix:
   include:
     - jdk: openjdk15
       env: JACOCO_SKIP="-Djacoco.skip=true"
+    - jdk: openjdk14
+    - jdk: openjdk11
+    - jdk: openjdk8
 
 script:
   - ./mvnw test -B $JACOCO_SKIP