fix check preventing range for string and bool parameters

[mikaelarguedas]

Noticed this when testing #111.

For example this generated successfully https://github.com/tuw-robotics/tuw_marker_detection/blob/a0a11bcd715a0928d4d4a0a295a82ed79014e69d/tuw_aruco/cfg/ArUco.cfg#L26

Now wit the check fixed:

Traceback (most recent call last):
  File "/home/mikael/work/ros/tuw_aruco_ws/src/tuw_marker_detection/tuw_aruco/cfg/ArUco.cfg", line 26, in <module>
    gen.add("marker_dictonary", str_t, 0, "Marker dictonary type", "ARTOOLKITPLUSBCH", "ARUCO", "TAG36h10", edit_method=marker_directory_enum)
  File "/home/mikael/work/ros/ros_base/install_isolated/lib/python2.7/dist-packages/dynamic_reconfigure/parameter_generator_catkin.py", line 290, in add
    self.group.add(name, paramtype, level, description, default, min, max, edit_method)
  File "/home/mikael/work/ros/ros_base/install_isolated/lib/python2.7/dist-packages/dynamic_reconfigure/parameter_generator_catkin.py", line 143, in add
    raise Exception("Max or min specified for %s, which is of string type" % name)
Exception: Max or min specified for marker_dictonary, which is of string type
CMakeFiles/tuw_aruco_gencfg.dir/build.make:63: recipe for target '/home/mikael/work/ros/tuw_aruco_ws/devel_isolated/tuw_aruco/include/tuw_aruco/ArUcoConfig.h' failed

[mikaelarguedas]

Retargeted at melodic-devel. Out of all indexed repos (all active ros distros) only tuw-robotics/tuw_marker_detection#2 was failing this test, so I think it's an acceptable risk to merge and release this in melodic (but not earlier distros).

[mikaelarguedas]

Prerelease passed on all released indigo dependents with 8cc924a