Switch Travis-CI to trusty and update jdk list

https://docs.travis-ci.com/user/reference/trusty/#jvm-clojure-groovy-java-scala-images

.travis.yml

@@ -1,14 +1,13 @@
 language: java
 sudo: false
-dist: bionic
+dist: trusty
 jdk:
   # It's important to test on all these because of a horrible hack in the POM.
-# - openjdk6  # No longer supported by Travis
+# - openjdk6
   - openjdk7
   - oraclejdk8
   - oraclejdk9
-  - openjdk10
-  - openjdk11
+  - oraclejdk11
 
 # Test each version with an appropriate Java version and flags to disable
 # GPG signing so that we don't have to provision Travis with private keys.