Drop ROS dependency on catkin

As a pure CMake package, ceres-solver doesn't need catkin to be built.
This change makes the package buildable using colcon as well.

Change-Id: I3c21f201f947a0e4e87da2708ab9e799816ba97a

package.xml

diff --git a/package.xml b/package.xml
index 62d97fb..9c7e04d 100644
--- a/package.xml
+++ b/package.xml
@@ -38,7 +38,7 @@
   <license>New BSD</license>
   <url type="website">http://ceres-solver.org/</url>
 
-  <buildtool_depend>catkin</buildtool_depend>
+  <buildtool_depend>cmake</buildtool_depend>
 
   <depend>atlas</depend>
   <depend>eigen</depend>