Fix AlgorithmFactory::unsubscribe method to updates version map.

It was previously not updating the version map when the algorithm was
removed and then subscribing the same thing again would throw an error.
Refs #7263

Code/Mantid/Framework/API/src/AlgorithmFactory.cpp

@@ -84,6 +84,17 @@ namespace Mantid
     try
     {
       Kernel::DynamicFactory<Algorithm>::unsubscribe(key);
+      // Update version map accordingly
+      VersionMap::iterator it = m_vmap.find(algorithmName);
+      if(it != m_vmap.end())
+      {
+        int highest_version = it->second;
+        if(highest_version > 1 && version == highest_version) // Decrement the highest version
+        {
+          it->second -= 1;
+        }
+        else m_vmap.erase(algorithmName);
+      }
     }
     catch(Kernel::Exception::NotFoundError&)
     {