Fix round error

Replaced the lines responsible for this error:
```
Warning: Failed prop type: Material-UI: variant="round" was renamed variant="circular" for consistency.
```

__tests__/src/components/WorkspaceAddButton.test.js

@@ -65,7 +65,7 @@ describe('WorkspaceAddButton', () => {
       const wrapper = createWrapper({ useExtendedFab: false });
 
       expect(extendedWrapper.find(Fab).props().variant).toBe('extended');
-      expect(wrapper.find(Fab).props().variant).toEqual('round');
+      expect(wrapper.find(Fab).props().variant).toEqual('circular');
     });
   });
 });

src/components/WorkspaceAddButton.js

@@ -30,7 +30,7 @@ export class WorkspaceAddButton extends Component {
           }
           className={classes.fab}
           classes={{ primary: classes.fabPrimary, secondary: classes.fabSecondary }}
-          variant={useExtendedFab ? 'extended' : 'round'}
+          variant={useExtendedFab ? 'extended' : 'circular'}
           onClick={() => { setWorkspaceAddVisibility(!isWorkspaceAddVisible); }}
         >
           {