requiring pytest 2.5.1 to match trusty package

package.xml

@@ -27,7 +27,7 @@
   <depend version_gte="1.5.2">python-six</depend>
   <depend version_gte="2.9.1">marshmallow</depend>
   <!-- we depend on these here just because we embed tests inside the package -->
-  <depend version_gte="2.8.0">python-pytest</depend>  <!-- version based on hypothesis requirements but trusty only has 2.5.1, which seems to work anyway... -->
+  <depend version_gte="2.5.1">python-pytest</depend>  <!-- hypothesis requirements say 2.8.0 but trusty only has 2.5.1, which seems to work anyway... -->
   <depend version_gte="3.0.1">hypothesis</depend>  <!-- backport as rospackage for indigo and jade, but system package from xenial -->
 
   <build_depend version_gte="0.10.0">roslint</build_depend>