Source ros setup before rosdep

I debugged with @k-okada and got to the conclusion that we should source ros setup before rosdep install.

Command `rosep install --ignore-src` ignores the packages which locate under ROS_PACKAGE_PATH.
So far, we call `rosep install --ignore-src` without setting any ROS_PACKAGE_PATH, and that means `--ignore-src` have no effect.
By this PR, `rosep install --ignore-src` ignores the package which is already installed by apt.

This PR is necessary for the repository, which installs apt package in the individual travis.yaml.
ex. https://github.com/start-jsk/rtmros_gazebo/blob/master/.travis.yml#L32