Fix a libcurl conflict on debian

... was preventing to install GDAL (libgdal1-dev)

fix https://github.com/ros/robot_model/issues/37

"The .deb should depend on `libcurl-ssl-dev` and not `libcurl4-openssl-dev`,
that conflicts with `libcurl4-gnutls-dev` which GDAL-dev depends on."

EDIT: libgdal1 -> libgdal1-dev