Changed dependency proper to branch master.

There are some incompatibilities in the current tagged release (v1.1)
and OTP 18. As soon as they are fixed, there will be another tagged
release on github.

See:
* proper-testing/proper#100
* proper-testing/proper#107

rebar.config

@@ -60,7 +60,7 @@
   {jsx, "1\.4\..*", {git, "git://github.com/talentdeficit/jsx", "v1.4.2"}},
 
   %% for testing
-  {proper, "1\.1", {git, "git://github.com/manopapad/proper", "v1.1"}},
+  {proper, "1\.1", {git, "git://github.com/manopapad/proper", {branch, "master"}}}, %"v1.1"}},
   {meck, "0.8.2", {git, "git://github.com/eproxus/meck.git", "dde7590"}}
  ]
 }.