User model minor changes

src/models/user.js

         type: String
     },
     email: {
-        type: String
+        type: String,
+        required: true
     },
     password: {
-        type: String
+        type: String,
+        required: true
     },
     tokens: [{
         token: {
             type: String,
-            required: true
         }
     }]
 })