Use JDK 11 to compile

This works around an issue in error-prone.
2021-03-20 00:34:40 +09:00 · 2021-03-20 00:34:40 +09:00 · 1506f482c1
commit 1506f482c1
--- a/.travis.yml
+++ b/.travis.yml
@ -3,7 +3,7 @@ version: ~> 1.0
 os: linux
 language: java
 jdk:
-  - openjdk8
+  - openjdk11
 env:
  global:
    - secure: "kLL/DJT13SO4WGpqzBTRqzbk4sOg6zeczQXSbC6I2W4hUCrr1hgFZt2bhZAI7AD89Q633Yj0XczOFWAlKIb0h/1jauCHwxmhDKieDI6VibnoauuFlKDT/279768Wg4oVnRM+uSeSd69jrsTnngbCckKAgSqow0XR3BcM6iSITaM="  # DOCKER_USER