fix jdk as new Xenial image on Travis doesn't support oraclejdk8. added new build for jdk13

Author: Pazus

Diff for: .travis.yml

@@ -4,7 +4,7 @@ services:
   - docker
 
 jdk:
-  - oraclejdk8
+  - openjdk8
 
 env:
   global:
@@ -42,6 +42,8 @@ matrix:
     jdk: openjdk11
   - env: UTPLSQL_VERSION="v3.1.7"
     jdk: openjdk12
+  - env: UTPLSQL_VERSION="v3.1.7"
+    jdk: openjdk13
 
 before_cache:
   - rm -f  $HOME/.gradle/caches/modules-2/modules-2.lock