Update rebar.config

rebar.config

@@ -1,7 +1,7 @@
 {deps_dir, ["deps"]}.
 {deps, [
     {n2o,           ".*", {git, "git://github.com/5HT/n2o", {tag,"1.10"}}},
-    {kvs,           ".*", {git, "git://github.com/synrc/kvs", "HEAD"}},
+    {kvs,           ".*", {git, "git://github.com/synrc/kvs", {tag,"2.1"}}},
     {erlydtl,       ".*", {git, "git://github.com/evanmiller/erlydtl.git", {tag,"0.8.0"}}}
 ]}.
 {erlydtl_opts, [