Update build tools and prepare for a new release

Author: elbrujohalcon

src/shotgun.app.src

@@ -2,7 +2,7 @@
  [
   {description,
    "better than just a gun"},
-  {vsn, "0.5.2"},
+  {vsn, "0.5.3"},
   {applications,
     [kernel,
      stdlib,
@@ -16,6 +16,6 @@
           , {"Docs", "http://inaka.github.io/shotgun/"}
           , {"SSE Spec", "https://html.spec.whatwg.org/multipage/comms.html#server-sent-events"}
           ]},
-  {build_tools, ["rebar"]}
+  {build_tools, ["rebar3"]}
  ]
 }.