Remove luxon and es6-object-assign

[San4es]

These libs were necessary for rrule v2.6.4 (see jkbrzt/rrule#344 and jkbrzt/rrule#402). But starting with v2.6.6 it is no need to include them (see https://github.com/jakubroztocil/rrule/releases/tag/v2.6.6 and jkbrzt/rrule#410).

This PR updates rrule to v2.6.6 and removes luxon and es6-object-assign libs from our dependencies as not necessary now.

[DokaRus]

@San4es Object.assign and extend works slightly different.
E.g. merge two objects: { name: 'a' } and { name: undefined }:

Won't there be an unexpected change in behavior?

[San4es]

@DokaRus

Thanks for your remark! I think we need to check this in each case.