core: Upcoming Zotonic release will require Erlang R15B03-1 or newer.

Rationale is to always support up to 2 major versions behind the
current (or upcoming) major version. At this point the upcoming Erlang
version is R17.
core: Downgrade required OTP release to R15B03

It turns out there are no binary packages available for R15B03-1, and a
lot of distros are still on plain R15B03.

Fixes #740