Switch to openjdk11 build (#100)

* Switch to openjdk11 build

* Upgrade to Gradle v5.5.1

.travis.yml

@@ -1,6 +1,6 @@
 language: java
 jdk:
-  - oraclejdk8
+  - openjdk11
 #  blacklist
 branches:
   except:

gradle/wrapper/gradle-wrapper.properties

@@ -3,4 +3,4 @@ distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-2.13-bin.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-5.5.1-bin.zip