When 'languageVersion' isn't passed explicitly, compiler will choose
'LATEST_STABLE'.
This may lead to spurious migration of a project to the next major
version after plugin update (e.g. when user had project of major
version X and then installed plugin of version X+1 -- in such case,
meaning of "no language version" suddenly changes).
So, we set it to RELEASED_VERSION, which is exactly the version
that is safe to use as the fallback.
Dmitry Savvinov
b23bf371cf