[PIO-214] use openjdk8 as oraclejdk8 is no longer available in the default Travis build image (#525)

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index f6037b5..8501bc1 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -30,7 +30,7 @@
 language: scala
 
 jdk:
-  - oraclejdk8
+  - openjdk8
 
 services:
   - docker