Revert afterInstall -> beforeInstall change. afterInstall is fine.

Author: chriskrycho

blueprints/ember-cli-typescript/index.js

@@ -24,7 +24,7 @@ module.exports = {
     // Entity name is optional right now, creating this hook avoids an error.
   },
 
-  beforeInstall: function() {
+  afterInstall: function() {
     return this.addPackagesToProject([
       { name: 'typescript', target: '^2.1' },
       { name: '@types/ember', target: '^2.7.34' }