Add support for Electron 5 and above

Starting with Electron 5.0.0, the function `webFrame.setSpellCheckProvider` has a different signature, in order to support asynchronous spell checkers. For more information on this change in Electron, see electron/electron#14032.

This diff updates `SpellCheckHandler` to use the new interface if running on Electron 5.0.0 and above. However, it still checks the spelling synchronously like before.

If running on Electron versions below 5.0.0, the `SpellCheckHandler` still works.

Closes electron-userland#144.

src/spell-check-handler.js

@@ -465,14 +465,10 @@ module.exports = class SpellCheckHandler {
    */
   setSpellCheckProvider(webFrame) {
     if (process.versions.electron >= '5.0.0') {
-      d('DEBUG: Setting provider for Electron 5');
-      // Electron 5 and above:
       webFrame.setSpellCheckProvider(
         this.currentSpellcheckerLanguage,
         { spellCheck: this.handleElectronSpellCheck.bind(this) });
     } else {
-      d('DEBUG: Setting provider for Electron 4');
-      // Electron 4 and below:
       webFrame.setSpellCheckProvider(
         this.currentSpellcheckerLanguage,
         this.shouldAutoCorrect,