Made POSIX compatible to work with dash

I noticed these errors when running Gazebo:

process[gazebo-2]: started with pid [12052]
/home/isura/catkin_ws/src/gazebo_ros_pkgs/gazebo_ros/scripts/gzserver: 5: [: Linux: unexpected operator
process[gazebo_gui-3]: started with pid [12061]
/home/isura/catkin_ws/src/gazebo_ros_pkgs/gazebo_ros/scripts/gzclient: 5: [: Linux: unexpected operator

It turns out that /bin/sh used to be a symlink to bash which is okay with "==" for string compare. In later versions of Debian/Ubuntu /bin/sh points to a lighter alternative to bash called dash. Dash is more strict in its POSIX compliance and thus will not accept "==" for string compare.

See references:
http://askubuntu.com/questions/141928/what-is-difference-between-bin-sh-and-bin-bash
http://stackoverflow.com/questions/1089813/bash-dash-and-string-comparison