version number in spec files is not correct for 1.5.x

[schristley]

Both of the AIRR schema spec files say "1.4" when should be "1.5".

Because the master branch has already moved significantly towards 2.0, I've created a release-1.5 branch starting from the v1.5.1 tag according to our release processes. This branch won't get merged to master but will stay in case minor fixes need to be done for the 1.5.x release chain.