Repository update detects changes in metadata

When a change in the metadata for a particular mod version is detected on repository update, CKAN will reinstall that mod. This allows us to make changes to the "install" sections of files in CKAN-meta without breaking anything.

After merging please also merge the client part of this patch:
https://github.com/KSP-CKAN/CKAN-cmdline/pull/27
https://github.com/KSP-CKAN/CKAN-GUI/pull/18