Use proper method to subscribe to onDidTokenize

Way back in
atom/atom@117a3fb
(released in Atom v1.9.0-beta0) Atom introduced an experimental
TextEditor::onDidTokenize method in order to subscribe to that event.
Switch to using this instead of grabbing it through the private
properties which no longer work as of Atom v1.25.0-beta0.

Fixes #649.

lib/minimap.js

@@ -270,11 +270,7 @@ class Minimap {
     resulting in extra lines appearing at the end of the minimap.
     Forcing a whole repaint to fix that bug is suboptimal but works.
     */
-    const tokenizedBuffer = this.textEditor.tokenizedBuffer
-      ? this.textEditor.tokenizedBuffer
-      : this.textEditor.displayBuffer.tokenizedBuffer
-
-    subs.add(tokenizedBuffer.onDidTokenize(() => {
+    subs.add(this.textEditor.onDidTokenize(() => {
       this.emitter.emit('did-change-config')
     }))
   }