Error deactivating package 'minimap-pigments' TypeError: Cannot read property 'dispose' of undefined

Noticed this while developing another plugin, triggered when reloading the window:

```
/Applications/Atom.app/Contents/Resources/app.asar/src/package.js:655 Error deactivating package 'minimap-pigments' TypeError: Cannot read property 'dispose' of undefined
at Object.module.exports.deactivate (/Users/viddo/.atom/packages/minimap-pigments/lib/minimap-pigments.coffee:24:25)
at Package.module.exports.Package.deactivate (/Applications/Atom.app/Contents/Resources/app.asar/src/package.js:644:21)
at PackageManager.module.exports.PackageManager.deactivatePackage (/Applications/Atom.app/Contents/Resources/app.asar/src/package-manager.js:622:12)
at /Applications/Atom.app/Contents/Resources/app.asar/src/package-manager.js:608:19
at Config.module.exports.Config.transact (/Applications/Atom.app/Contents/Resources/app.asar/src/config.js:312:16)
at PackageManager.module.exports.PackageManager.deactivatePackages (/Applications/Atom.app/Contents/Resources/app.asar/src/package-manager.js:602:19)
at AtomEnvironment.module.exports.AtomEnvironment.unloadEditorWindow (/Applications/Atom.app/Contents/Resources/app.asar/src/atom-environment.js:799:21)
at WindowEventHandler.module.exports.WindowEventHandler.handleWindowBeforeunload (/Applications/Atom.app/Contents/Resources/app.asar/src/window-event-handler.js:196:30)
at /Applications/Atom.app/Contents/Resources/app.asar/src/window-event-handler.js:3:61module.exports.Package.deactivate @ /Applications/Atom.app/Contents/Resources/app.asar/src/package.js:655module.exports.PackageManager.deactivatePackage @ /Applications/Atom.app/Contents/Resources/app.asar/src/package-manager.js:622(anonymous function) @ /Applications/Atom.app/Contents/Resources/app.asar/src/package-manager.js:608module.exports.Config.transact @ /Applications/Atom.app/Contents/Resources/app.asar/src/config.js:312module.exports.PackageManager.deactivatePackages @ /Applications/Atom.app/Contents/Resources/app.asar/src/package-manager.js:602module.exports.AtomEnvironment.unloadEditorWindow @ /Applications/Atom.app/Contents/Resources/app.asar/src/atom-environment.js:799module.exports.WindowEventHandler.handleWindowBeforeunload @ /Applications/Atom.app/Contents/Resources/app.asar/src/window-event-handler.js:196(anonymous function) @ /Applications/Atom.app/Contents/Resources/app.asar/src/window-event-handler.js:3
```

https://github.com/abe33/minimap-pigments/blob/master/lib/minimap-pigments.coffee#L24 `@editorsSubscription` does not always exist, it seems