chore: remove openjdk11 from the build matrix

Travis CI currently fails to install OpenJDK 11. Apparently the install
script is quite brittle and breaks whenever a new build is uploaded and
Oracle removes the old download page.

See https://travis-ci.community/t/install-of-openjdk11-is-failing/1914

Let's keep track of this issue on Travis side, and re-add `openjdk11`
as soon as it is fixed.

.travis.yml

@@ -1,6 +1,5 @@
 language: java
 jdk:
- - openjdk11
  - oraclejdk11
  - openjdk10
  - oraclejdk9