Removing double quotes, adding admin import

Removing any double quotes and adding an admin
import statement to the top of the file to resolve
Travis CI build failure.

Author: jenperson

auth/create_custom_tokens.js

@@ -1,3 +1,6 @@
+'use strict';
+const admin = require('firebase-admin');
+
 // Initialize the Admin app with the default appication credentials
 // [START initialize_sdk_with_default_config]
 admin.initializeApp();
@@ -11,28 +14,28 @@ admin.initializeApp({
 // [END initialize_sdk_with_service_account_id]
 
 // [START custom_token]
-var uid = "some-uid";
+var uid = 'some-uid';
 
 admin.auth().createCustomToken(uid)
   .then(function(customToken) {
     // Send token back to client
   })
   .catch(function(error) {
-    console.log("Error creating custom token:", error);
+    console.log('Error creating custom token:', error);
   });
 // [END custom_token]
 
 // [START custom_token_with_claims]
-var uid = "some-uid";
+var userId = 'some-uid';
 var additionalClaims = {
   premiumAccount: true
 };
 
-admin.auth().createCustomToken(uid, additionalClaims)
+admin.auth().createCustomToken(userId, additionalClaims)
   .then(function(customToken) {
     // Send token back to client
   })
   .catch(function(error) {
-    console.log("Error creating custom token:", error);
+    console.log('Error creating custom token:', error);
   });
 // [END custom_token_with_claims]