fix compile error on OS X

Without the file suffix you get the following error on OS X.

Submodule path 'deps/meck': checked out '1286aba1cb6bbd6c9fc3f817740758b224843ee
7'
cd deps/meck \
                && cp rebar.config rebar.config.bak \
                && sed -i 's/warnings_as_errors, //' rebar.config \
                && make get-deps \
                && make compile \
                && mv rebar.config.bak rebar.config
sed: -i may not be used with stdin
make: *** [meck] Error 1

I tested this change on OS X and Linux and it seems to do the right thing.  Most likely you could change '.bk' to '.bak' and remove the 'cp' line but this will get the tool to compile and run.