chore: use openjdk7 to boostrap Travis CI images for JDK 6 (#1366)
chore: use openjdk7 to boostrap Travis CI images for JDK 6 (#1366)

In fact, Zulu JDK 6 is used for Java 6, so `jdk: openjdk7` does not matter much
marschall/pgjdbc,golovnin/pgjdbc,marschall/pgjdbc,pgjdbc/pgjdbc,pgjdbc/pgjdbc,jorsol/pgjdbc,AlexElin/pgjdbc,jorsol/pgjdbc,davecramer/pgjdbc,golovnin/pgjdbc,marschall/pgjdbc,AlexElin/pgjdbc,jorsol/pgjdbc,davecramer/pgjdbc,AlexElin/pgjdbc,sehrope/pgjdbc,marschall/pgjdbc,sehrope/pgjdbc,pgjdbc/pgjdbc,jorsol/pgjdbc,sehrope/pgjdbc,sehrope/pgjdbc,golovnin/pgjdbc,davecramer/pgjdbc,pgjdbc/pgjdbc,davecramer/pgjdbc