The official remote for rebar is now rebar, not basho

Same tags should be there, but the rebar remote has more recent releases
that projects may wish to use via override (saving update of default
version for a later commit).
Add relx and up default_version of rebar to 2.2.0

relx provides more convenient Erlang project release generation, but
depends on rebar > 2.2.0 to we're upping the default rebar version to
the SHA of current HEAD.
Merge branch 'sf/add-relx'