Fixed the wiki not being updated on tagged releases

https://travis-ci.org/OpenRA/OpenRA/builds/36429861 did not run it, because I told Travis so, but this is in fact not what we want here.
I still think it is inappropriate to overwrite our release documentation (which >95% of games are played on, making it the only sensible branch for mappers to target) with playtest versions.  You can copy this to `Lua-API-(playtest).md` instead if `$TRAVIS_BRANCH == "next"`.