Upgrade rebar

In order to make the build work on Windows a more recent rebar is needed.
Rebar is upgraded to:

    rebar 2.5.1 R14B04 20140730_173812 git 2.5.1-1-ge9f62c4

Change-Id: I8416aa5fea727ef7f7148fd06090794ec99d19ff
Reviewed-on: http://review.couchbase.org/40493
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>