Replace alias with alias_method

Done with `rubocop --auto-correct --only Alias`

lib/fog/cloudsigma/models/mountpoint.rb

@@ -18,7 +18,7 @@ module Fog
         def drive=(new_drive)
           attributes[:drive] = new_drive
         end
-        alias :volume :drive
+        alias_method :volume, :drive
       end
     end
   end

lib/fog/cloudsigma/models/server.rb

@@ -60,7 +60,7 @@ module Fog
           true
         end
 
-        alias :delete :destroy
+        alias_method :delete, :destroy
 
         def start(start_params={})
           requires :identity

lib/fog/cloudsigma/models/vlan.rb

@@ -22,7 +22,7 @@ module Fog
           merge_attributes(new_attributes)
         end
 
-        alias :save :update
+        alias_method :save, :update
       end
     end
   end

lib/fog/cloudsigma/models/volume.rb

@@ -55,7 +55,7 @@ module Fog
           true
         end
 
-        alias :delete :destroy
+        alias_method :delete, :destroy
 
         def clone(clone_params={})
           requires :identity