Fixed an embarrassing bug in the load path provider for the Coq library

Commit ec766686938855343de5bb02cd8318e82b444001 ported the old load path
code to the then-new abstract load path mechanism, introducing a silly
mistake in the process: the directory for users' third-party extensions is
called "user-contrib", not "user-theories"
Bumped the version number to 0.3.7.1 and added a changelog entry

This is the first humiliating bugfix release of the 0.3.x series. Hooray!