fetch a specific version of rebar in the makefile

[hyperthunk]

especially useful for travis, where we do not control the version of rebar used; works nicely in SysTest:

REBAR=bin/rebar

compile: $(REBAR)
    $(REBAR) get-deps compile -v $(LOGLEVEL)


bin/%:
    mkdir -p deps
    mkdir -p bin
    git clone -b systest https://github.com/$*/$*.git deps/$*
    PATH="bin:${PATH}" $(MAKE) -C deps/$*
    cp deps/$*/$* bin/$*