stix2-validator version is wrong. (continued from #11)

[stmtstk]

I found when I setup S-TIP RS, the latest stix2-validator is installed.
I checked the dependencies,

https://github.com/oasis-open/cti-stix-elevator

stix-elevator depends on stix2-validator.

So, we should also modify requirements_rs.txt.
We should add "stix-validator==2.0.0.dev1" below "stix2-elevator".

[stmtstk]

@ykoji8681 if you are not hurry, I want to check this installation tomorrow.