Adjust Travis-CI build to properly build Github Pull Requests

We need to install a newer version of Ant
Also checkout documentation-repository
Invoke target "jenkins" to run all checks
Switch to base image for Ubuntu Bionic

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1884167 13f79535-47bb-0310-9956-ffa450edef68

.travis.yml

@@ -1,8 +1,14 @@
+dist: bionic
 language: java
 jdk:
 - openjdk11
-before_install: /bin/true
+before_install:
+  - wget https://www.apache.org/dist/ant/binaries/apache-ant-1.10.9-bin.tar.gz
+  - tar -xzvf apache-ant-1.10.9-bin.tar.gz
+  - export PATH=`pwd`/apache-ant-1.10.9/bin:$PATH
+  - echo $(ant -version)
+  - svn co https://svn.apache.org/repos/asf/poi/site/src/documentation src/documentation
 install: /bin/true
 before_script: /bin/true
 script:
-- ant clean test rat-check
+- ant clean jenkins