Switch from double to singe quotes to enable parameter expansion

.travis.yml

@@ -24,10 +24,10 @@ before_install:
 
 script:
   - docker run --name nav2_bash --rm -d -i -t nav2:latest bash -l
-  - docker exec nav2_bash bash -c ". /opt/ros/$ROS2_DISTRO/setup.bash &&
+  - docker exec nav2_bash bash -c '. /opt/ros/$ROS2_DISTRO/setup.bash &&
       . /ros2_ws/ros2-linux/setup.bash &&
       . /ros2_ws/navstack_dependencies_ws/install/setup.bash &&
       colcon test --packages-skip dwb_controller dwb_core nav2_amcl nav2_costmap_2d
         nav2_dynamic_params nav2_map_server nav2_navfn_planner nav2_robot nav2_system_tests
-        nav2_tasks nav2_util"
+        nav2_tasks nav2_util'
   - docker exec nav2_bash bash -c "colcon test-result --verbose"