Track pystac versions

[moradology]

Pystac generated catalogs are completely incompatible between minor versions at the moment and development efforts (aimed at 0.4) have required us to write Dockerfiles to extend the RV images (which use 0.3). This will be simpler once things stabilize a bit more in pystac but it might be sensible to explicitly track pystac versions in the requirements.txt for this project's Dockerfile

[AdeelH]

Pystac is being tracked in https://github.com/azavea/raster-vision/blob/master/rastervision_core/requirements.txt so closing this.