Update gazebo_ros_gps.cpp

corrected referenceAltitude and referenceHeading parameter readings
Merge pull request #1 from rlklaser/groovy-devel

hector_gazebo_plugins: fixed referenceHeading/referenceAltitude parameter naming in gazebo_ros_gps.cpp
Merge remote-tracking branch 'origin/groovy-devel' into hydro-devel