Set @level_override to fix Ruby HEAD compatibility

ruby/logger#85 added this to the `Logger` initializer.

Since we don't call `super` in `initialize`, we need to do it ourselves.

Not doing this leads to an error in `Logger#level`:

```ruby

def level
  @level_override[Fiber.current] || @Level
  #              ^^^^^^^^^^^^^^^
end
```

lib/mono_logger.rb

@@ -30,6 +30,7 @@ class MonoLogger < Logger
   def initialize(logdev, shift_age=nil, shift_size=nil)
     @progname = nil
     @level = DEBUG
+    @level_override = {}
     @default_formatter = Formatter.new
     @formatter = nil
     @logdev = nil