Use the Git protocol for PropEr

I've had reports that HTTPS seems to fail a bit more often than the Git
URLs. I've seen this before as well so I've gone ahead and switched
protocols for the clone.

rebar.config

@@ -1,5 +1,5 @@
 {deps, [
-    {proper, ".*", {git, "https://github.com/manopapad/proper.git", "master"}}
+    {proper, ".*", {git, "git://github.com/manopapad/proper.git", "master"}}
 ]}.
 
 {port_specs, [{"priv/jiffy.so", ["c_src/*.c"]}]}.