travis: also build release branch

Also setup travis to clone the corresponding branch of AutomataLib, so that
e.g. building the release branch also uses the release branch of AutomataLib.

(cherry picked from commit d513063)

Author: mtf90

.travis.yml

@@ -17,8 +17,13 @@ branches:
   only:
   - master
   - develop
+  - release
+
 before_install:
-  - git clone -b develop --single-branch https://github.com/LearnLib/automatalib.git /tmp/automatalib-develop
+    # set Travis environment variables "AUTOMATALIB_FORK" and "AUTOMATALIB_BRANCH" to build custom AutomataLib versions.
+    # Defaults are "LearnLib" and the current/targeted LearnLib branch ($TRAVIS_BRANCH, relies on the same naming
+    # conventions between AutomataLib and LearnLib branches).
+  - git clone -b ${AUTOMATALIB_BRANCH:-$TRAVIS_BRANCH} --single-branch https://github.com/${AUTOMATALIB_FORK:-LearnLib}/automatalib.git /tmp/automatalib-develop
   - pushd /tmp/automatalib-develop
     # skip several aspects of the build process, because we are only interested in the compiled code
   - mvn install -DskipTests -Dmaven.javadoc.skip=true