Fix: Occurrence#overnight? did not work for start times on the last day of a month

When the `overnight?` method was called on an occurrence with a start time on the last day of a month, it threw an `argument out of range` exception.
```
[1] pry(main)> occurrence = IceCube::Occurrence.new Time.new(2014, 3, 31)
=> 2014-03-31 00:00:00 +0200
[2] pry(main)> occurrence.overnight?
ArgumentError: argument out of range
from /Users/phil/.rvm/gems/ruby-2.0.0-p353@iq/gems/ice_cube-0.11.3/lib/ice_cube/occurrence.rb:96:in `initialize'
```