journey test: fix for trackler submodule rename

https://github.com/exercism/trackler/commit/69e055ad52b3f8d19da95b6ab0593678b8ee1bb4

bin/journey-test.sh

   local version=$( grep -m 1 'trackler' ${xapi_home}/Gemfile.lock | sed 's/.*(//' | sed 's/)//' )
 
   git checkout v${version}
-  git submodule init -- common
+  git submodule init -- problem-specifications
   git submodule update
 
   # Bake in local copy of this track; this is what we are testing.