Upgrade JDK to openjdk10

This is because
```
Compile, test and install all
=============================
/home/travis/build/jflex-de/jflex/cup-maven-plugin/src/main/java/jflex/maven/plugin/cup/GenerateMojo.java:5: error: cannot access BufferedReader
import java.io.BufferedReader;
              ^
  bad class file: /modules/java.base/java/io/BufferedReader.class
    class file has wrong version 55.0, should be 53.0
    Please remove or make sure it appears in the correct subdirectory of the classpath.
1 error
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.7.0:compile (default-compile) on project cup-maven-plugin: Compilation failure -> [Help 1]
```

https://travis-ci.org/jflex-de/jflex/jobs/607336245?utm_medium=notification&utm_source=github_status

.travis.yml

@@ -12,7 +12,7 @@ git:
 language: java
 
 jdk:
-- openjdk9
+- openjdk10
 
 matrix:
   include: