Adjusted rtl classname to be modifier

[arsdehnel]

updated the classname things requested in JedWatson#1613, being pretty new to contributing to OSS I think I got this right that I can just do this on your fork and then that'll get updated over to your PR on the main repo. If I got that wrong let me know, just trying to help finish this up since it's clear you've put in a lot of work here and it's very close. Would love to have RTL support on this component.

[dekelb]

@arsdehnel thanks for this PR!

The only problem I see there is that it will break the rtl test which you can find here:
JedWatson@1d39026

You are welcome to update this PR with the relevant updated tests and I'll be happy to approve it.

I'm afraid the only reason I didn't take the time to fix it is because the react-select looks abandoned. The last release there was v1.0.0-rc.5 (on May) and there are many bugs that still weren't fixed (you can see for example my PR here: JedWatson#1612 which is a real bug that is
still open with a good PR since Mar).

I have to say that I was really disappointed... when I was checking Select components for react it was one of the best I could find, and the current status of this lib is really not good.
I even thought on starting a new project for a better react select component...