Updated rubocop and fixed issues (#490)

app/models/room.rb

@@ -67,9 +67,9 @@ class Room < ApplicationRecord
       unless meeting[:messageKey] == 'duplicateWarning'
         update_attributes(sessions: sessions + 1, last_session: DateTime.now)
       end
-    rescue BigBlueButton::BigBlueButtonException => exc
-      puts "BigBlueButton failed on create: #{exc.key}: #{exc.message}"
-      raise exc
+    rescue BigBlueButton::BigBlueButtonException => e
+      puts "BigBlueButton failed on create: #{e.key}: #{e.message}"
+      raise e
     end
   end
 

app/models/user.rb

@@ -225,7 +225,7 @@ class User < ApplicationRecord
 
   # Initializes a room for the user and assign a BigBlueButton user id.
   def initialize_main_room
-    self.uid = "gl-#{(0...12).map { (65 + rand(26)).chr }.join.downcase}"
+    self.uid = "gl-#{(0...12).map { rand(65..90).chr }.join.downcase}"
     self.main_room = Room.create!(owner: self, name: I18n.t("home_room"))
     save
   end