release: Update package.xml version to 3.1.0

package.xml

 <?xml version="1.0"?>
 <package format="3">
   <name>eigenpy</name>
-  <version>3.0.0</version>
+  <version>3.1.0</version>
   <description>Bindings between Numpy and Eigen using Boost.Python</description>
   <maintainer email="justin.carpentier@inria.fr">Justin Carpentier</maintainer>
   <maintainer email="opensource@wolfgangmerkt.com">Wolfgang Merkt</maintainer>