Use OpenJDK on Travis. (#368)

Also specify an explicit Travis dist to avoid any auto-update breaking the build.

Author: TATSUNO Yasuhiro

.travis.yml

@@ -1,4 +1,4 @@
-sudo: false
+dist: xenial
 language: scala
 script:
   - sbt ++$TRAVIS_SCALA_VERSION package doc example/compile
@@ -9,7 +9,7 @@ scala:
   - 2.12.8
   - 2.13.0-RC1
 jdk:
-  - oraclejdk8
+  - openjdk8
 env:
   - SCALAJS_VERSION=0.6.27
   - SCALAJS_VERSION=1.0.0-M7