The -devel package should represent the latest stable version (i.e the one we are currently building against).
You are correct that the major number (the ABI version) is added to the final library name so that older versions can be kept in the repo to cater for dependent packages that haven't been rebuilt against the new ABI version (ideally they all should be rebuilt and the old library can then be removed to stop the repo getting cluttered with old versions). Without that difference in package name apt/synaptic would remove the old library and replace it with the new during an update resulting in broken packages. All new packages should be being built against the latest stable ABI version which is why the -devel package does not generally have the major number added to the name.
You don't need to do anything to the SPEC file. You specify libtorrent-rasterbar-devel in the Buildrequires and that will install (currently) libtorrent-raterbar-devel-0.15.8-1
(which is for ABI version 6) which means you are building against the currently supported version. If a package won't (or can't be patched to) build against the current version it is probably not worth having