Update rebar to 2.1.0-pre-168-gaa46d85

./rebar --version
rebar 2.1.0-pre R14B04 20131017_164918 git 2.1.0-pre-168-gaa46d85
Add dependency tree graph Makefile target
Correct graphviz node names with hyphen

h/t @shino
Merge pull request #711 from basho/feature/update-rebar-and-add-deps-graphings

Add dependency graphing