### Time intervals between attempts

For fixed intervals, an `:interval` parameter can be passed indicating the time in seconds.
The following example will sleep two seconds between attempts:

    retry_upto(5, :interval => 2)

For customized intervals, the `:interval` parameter can be a lambda, which will be applied
to the number of each attempt. For instance, the following code will sleep 3 seconds after
the first attempt, 6 after the second, 9 after the third...

    retry_upto(5, :interval => lambda{ |attempt| attempt * 3 })