ci: Switch to rawhide

While F26 satisfies all our build dependencies (not least because there's little to build), rawhide is much closer to the platform we are actually developing for, in particular when it comes to gjs and Spidermonkey.

Also rawhide's mozjs52-devel package now includes the js52 binary, so we can run the basic syntax check added in commit 65a20993 during continuous integration.