Fixing failing build because travis was using master branch of the lessons instead of develop

.travis.yml

@@ -6,6 +6,7 @@ install: "/bin/true"
 script:
   - mvn clean install
   - git clone https://github.com/WebGoat/WebGoat-Lessons.git
+  - git checkout develop
   - mvn -file ./WebGoat-Lessons/pom.xml clean package
   - cp -fa ./WebGoat-Lessons/target/plugins/*.jar ./webgoat-container/src/main/webapp/plugin_lessons/
   - if [[ $TRAVIS_PULL_REQUEST == "false" ]]; then mvn "-Dbuild.number=$TRAVIS_BUILD_NUMBER" -Prun-integration-tests clean install; else mvn clean install; fi