test on OpenJDK 8 and 11

`oraclejdk8` doesn't work anymore on Travis-CI, so PR validation
wasn't working

and then adding 11 is just a good thing in general

Author: SethTisue

.travis.yml

@@ -7,7 +7,8 @@ cache:
     - $HOME/.sbt/boot/
 
 jdk:
-  - oraclejdk8
+  - openjdk8
+  - openjdk11
 
 script:
   ## This runs the template with the default parameters, and runs test within the templated app.

project/build.properties

@@ -1 +1 @@
-sbt.version=0.13.17
+sbt.version=0.13.18