Switch to underscores for setup.cfg. (ros2#25) (#5)

This avoids a warning on newer setuptools.

Signed-off-by: Chris Lalancette <clalancette@openrobotics.org>

Co-authored-by: Chris Lalancette <clalancette@gmail.com>

setup.cfg

@@ -1,4 +1,4 @@
 [develop]
-script-dir=$base/lib/teleop_twist_keyboard
+script_dir=$base/lib/teleop_twist_keyboard
 [install]
-install-scripts=$base/lib/teleop_twist_keyboard
+install_scripts=$base/lib/teleop_twist_keyboard