stb-tester.spec: Add release number to version number

As now we have many custom releases, it's misleading to call all of them
"0.18-youview". It's possible to track the release number by checking
the version of the installed rpm, but it's neater to add it to `stbt -v`
output too.

The tags should be called "0.18.<release>-youview".
fixup! Makefile: Build stb-tester.rpm