Don't do StringVectorAttribute stuff for older OpenEXR versions

Solves build breaks for people with older OpenEXR.

OK, technically, OpenEXR 1.7 (but not older) supported StringVectorAttribute. But this code was very recently added, so I don't think it'll break any of those 1.7 users to disable it, and it's a LOT easier to identify when we're using >= 2.0. So I'm going to be lazy rather than tie myself in knots to distinguish between 1.6 and 1.7.