[AIRFLOW-XXX] Switch to openjdk8 in Travis tests

This is what we use (via our docker container) on master now, and either
Oracle are having problems with their download servers, or they are
playing silly games and have removed old versions for download.

Author: ashb

.travis.yml

@@ -20,7 +20,7 @@ sudo: true
 dist: trusty
 language: python
 jdk:
-  - oraclejdk8
+  - openjdk8
 services:
   - cassandra
   - mysql
@@ -92,7 +92,7 @@ before_install:
   - cat ~/.ssh/id_rsa.pub >> ~/.ssh/authorized_keys
   - ln -s ~/.ssh/authorized_keys ~/.ssh/authorized_keys2
   - chmod 600 ~/.ssh/*
-  - jdk_switcher use oraclejdk8
+  - jdk_switcher use openjdk8
 install:
   - pip install --upgrade pip
   - pip install tox