release: Update package.xml version to 5.1.0

package.xml

 <?xml version='1.0'?>
  <package format='2'>
  <name>hpp-manipulation</name>
- <version>5.0.0</version>
+ <version>5.1.0</version>
  <description>Classes for manipulation planning. </description>
 
  <maintainer email='hpp@laas.fr'>Joseph Mirabel</maintainer>