Time#in_milliseconds with millisecond accuracy

`Time` stores the number of nanoseconds since the Epoch.
`Time#in_milliseconds` should return the number of milliseconds since the
Epoch with a millisecond accuracy.
Ruby converts `1000` to `1000.0` to perform the multiplication. I believe it is more efficient to use `1000.0` then. See: http://www.ruby-doc.org/core-2.1.2/Float.html#method-i-2A