[5.8] Allow hourlyAt() to accept array of integers

[jrebs]

This small PR expands the functionality of the ManagesFrequencies::hourlyAt() method to allow it to accept an array of integers instead of only a single integer. FrequencyTest class includes passing test for this addition.

This is a non-breaking enhancement because the hourlyAt() method does not typehint the offset value being passed in.

Benefit to users: finer control over granular execution schedules. The hourlyAt() method is not chainable, so in the follow example, the command would only be executed at the 45th minute:

$schedule->command('my:command')->hourlyAt(15)->hourlyAt(30)->hourlyAt(45)

With this PR, it is possible to declare whatever hourly granularity is needed for your use case:

$schedule->command(my:command')->hourlyAt([15, 30, 45]);