nodelet-core: updating to 1.8.1

The nodelet-core recipes are updated to 1.8.1 to be in line with
https://github.com/ros/rosdistro/blob/4551e7c32d1b34984410b79bc0ec36afe3d3d245/hydro/release.yaml.

recipes-ros/nodelet-core/nodelet-core.inc

 SRC_URI = "https://github.com/ros/${ROS_SPN}/archive/${PV}.tar.gz;downloadfilename=${ROS_SP}.tar.gz"
-SRC_URI[md5sum] = "b4ec524761d1d2494f5901b77ad35f9b"
-SRC_URI[sha256sum] = "ad79f48e0916490d2095ea9ea81a796a060553d4e60ba5b148b96a95369b8124"
+SRC_URI[md5sum] = "fea2c22bff4933de722b67cc226c899a"
+SRC_URI[sha256sum] = "41aaadd0bb2378bae9a788839431546cfccc9f6b4cd8fc34684da1bb3d4356e1"
 
 S = "${WORKDIR}/${ROS_SP}/${ROS_BPN}"