Bump gem version to 0.8.0

Since dropping support for 2.7.5 in main (#30 (closed)) is, in theory, a major change, we should probably pip a minor release. There really isn't anything else to bundle, since nothing else has happened since 0.7.0 was cut, but we wouldn't want to accidentally bundle it into a 0.7.1 release.

..or maybe dropping support for 2.7.5 doesn't really warrant a full minor version, and we can just sneak in a 0.7.1. WDYT?