Fix issue #41: Hang up when "mathn" is required

The following code buzzed:

```ruby
require 'timers'
require 'mathn'

timers = Timers::Group.new
t = timers.after(5) { puts "OK!" }
sleep(1)
t.reset
timers.wait()
```
Merge pull request #42 from LavirtheWhiolet/patch-1

Fix issue #41: Hang up when "mathn" is required