Update extension.js

Modification for making work on Cinnamon 1.8.0.
panelMenu.js seems to do not exist anymore in Cinnamon 1.8.0 and replaced by panel.js (checked on Cinnamon git repository).

const PanelMenu = imports.ui.panelMenu;
replaced by : 
const PanelMenu = imports.ui.panel;
Update extension.js

panelMenu.js do not exist anymore in Cinnamon 1.8.X. 
So the import is removed.
Merge pull request #16 from TransposableBeuss/patch-1

Update extension.js