Update to version from 2018.03.31 (non-release)
    
    rosdep definitions describe libg2o-release to have its latest version
    taken upstream on the 2018.03.25 (https://github.com/ros-gbp/libg2o-release/releases).
    Hence take a version matching a similar date and modify the patch to
    adhere to upstream's c681c25.