[travis-ci] try specifying jdk: explicitly as that include is complaining
[travis-ci] try specifying jdk: explicitly as that include is complaining

```
$ jdk_switcher use ["openjdk7"]

Sorry, but JDK '[openjdk7]' is not known.
```

kares/activerecord-jdbc-adapter,jruby/activerecord-jdbc-adapter,keeguon/activerecord-jdbc-adapter,jruby/activerecord-jdbc-adapter,keeguon/activerecord-jdbc-adapter,kares/activerecord-jdbc-adapter,jruby/activerecord-jdbc-adapter,bruceadams/activerecord-jdbc-adapter,bruceadams/activerecord-jdbc-adapter,keeguon/activerecord-jdbc-adapter,bruceadams/activerecord-jdbc-adapter,kares/activerecord-jdbc-adapter