Installing_plugins.txt

mkdir -p ~/catkin_ws/src
cd ~/catkin_ws/src
catkin_init_workspace
cd ~/catkin_ws
catkin_make
echo "source ~/catkin_ws/devel/setup.bash" >> ~/.bashrc
sudo apt-get install git (if git is not installed.)
sudo apt-get install -y libgazebo7-dev
cd ~/catkin_ws/src
git clone https://github.com/ros-simulation/gazebo_ros_pkgs.git -b kinetic-devel

After this step put gazebo_ros_moveit_planning_scene.h file to: 
~/catkin_ws/src/gazebo_ros_pkgs/gazebo_plugins/include/gazebo_plugins

And put gazebo_ros_moveit_planning_scene.cpp to:
~/catkin_ws/src/gazebo_ros_pkgs/gazebo_plugins/src

Change your CMakelists.txt and package.xml with the updated ones.

Then:
rosdep update
rosdep check --from-paths . --ignore-src --rosdistro kinetic
rosdep install --from-paths . --ignore-src --rosdistro kinetic -y
cd ~/catkin_ws/
catkin_make


This should create gazebo_ros_moveit_planning_scene.so file which is used as the plugin.

EXTRA: 
To make robot's position in gazbo and rviz same: Change the part starts with <joint name="world_joint" type="fixed"> in ~/<your ws>/src/universal_robot/ur_description/urdf/ur5_gripper.xacro  file as:
<joint name="world_joint" type="fixed">
    <parent link="world" />
    <child link = "base_link" />
    <origin xyz="0.0 0.0 0.96" rpy="0.0 0.0 0.0" />
</joint>

And change the part starts with <node name="spawn_gazebo_model" in ~/<your ws>/src/universal_robot/ur_gazebo/launch/ur5.launch as:
<node name="spawn_gazebo_model" pkg="gazebo_ros" type="spawn_model" args="-urdf -param robot_description -model robot" respawn="false" output="screen" />