Travis: Use carl tag 18.08 to avoid problems with C++17

travis/build_carl_helper.sh

@@ -19,6 +19,7 @@ run() {
   travis_fold start install_carl
   git clone https://github.com/smtrat/carl.git
   cd carl
+  git checkout 18.08
   mkdir build
   cd build
   cmake .. "${CMAKE_ARGS[@]}"