Update Fedora / fcrepo to latest version within the 4.x branch

Turns out that since we're using the ualbertalib/docker-fcrepo4:4.7 docker image for Fedora/fcrepo (https://github.com/ualbertalib/docker-fcrepo4/blob/master/4.7/Dockerfile), we're not using the latest fcrepo version 4.7.5 within the 4.x version branch, but only version 4.7.3. Here's a list of changes between the two versions:

https://github.com/fcrepo/fcrepo/compare/fcrepo-4.7.3...fcrepo-4.7.5

And a list of reverted commits:

https://github.com/fcrepo/fcrepo/compare/fcrepo-4.7.5...fcrepo-4.7.3

I suspect that us using fcrepo 4.7.3 instead of 4.7.5 might also be a possible cause for some of the Ldp:HttpError issues we've been seeing, because some of these commits relate to LDP, for example this commit:

https://github.com/fcrepo/fcrepo/commit/7262c5feccbbb45e4ef0af35845d3ec7d4f71525

The commit message also refers to this issue, where some of the symptoms described look quite similar to issues we've been seeing on our RDMS instances:

https://fedora-repository.atlassian.net/browse/FCREPO-2378

Since the ualbertalib/docker-fcrepo4 docker image has not received any updates since february 2018, we should probably fork it and update it ourselves to 4.7.5, at least until we get the migration to fcrepo 6.x sorted out.