Remove Java 11 build because of a compiler bug #1519

Java 11's javac fails with:

[INFO] Compiling 669 source files to /home/travis/build/lettuce-io/lettuce-core/target/classes
compiler message file broken: key=compiler.misc.msg.bug arguments=11.0.2, {1}, {2}, {3}, {4}, {5}, {6}, {7}
java.lang.AssertionError
	at jdk.compiler/com.sun.tools.javac.util.Assert.error(Assert.java:155)
	at jdk.compiler/com.sun.tools.javac.util.Assert.check(Assert.java:46)
	at jdk.compiler/com.sun.tools.javac.comp.DeferredAttr$2$1.setOverloadKind(DeferredAttr.java:172)
	at jdk.compiler/com.sun.tools.javac.comp.ArgumentAttr.visitReference(ArgumentAttr.java:283)
	at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCMemberReference.accept(JCTree.java:2190)
	at jdk.compiler/com.sun.tools.javac.comp.ArgumentAttr.attribArg(ArgumentAttr.java:197)
	at jdk.compiler/com.sun.tools.javac.comp.Attr.attribTree(Attr.java:653)

.travis.yml

@@ -30,8 +30,6 @@ jobs:
       env:
         - JDK='OpenJDK 8'
         - REDIS=3.2
-    - env: JDK='OpenJDK 11'
-      before_install: wget https://github.com/sormuras/bach/raw/master/install-jdk.sh && . ./install-jdk.sh -f 11
     - env: JDK='OpenJDK 15'
       before_install: wget https://github.com/sormuras/bach/raw/master/install-jdk.sh && . ./install-jdk.sh -f 15
     - env: JDK='OpenJDK EA'