fix some build issues

I have no idea if this project is dead but it seems some build
issues need fixing so this is my first pass at doing so.

.travis.yml

@@ -1,19 +1,19 @@
 sudo: required
+services:
+ - xvfb
 language: java
 
 jdk:
- - oraclejdk8
- - oraclejdk9
- - openjdk7
+ - oraclejdk11
+ - openjdk8
+ - openjdk11
 
 before_install:
  - sudo apt-get -qq update
  - sudo apt-get build-dep libjlatexmath-java
 
 before_script:
  - "export DISPLAY=:99.0"
- - "sh -e /etc/init.d/xvfb start"
- - sleep 3 # give xvfb some time to start
 
 after_success:
  - mvn cobertura:cobertura coveralls:report