upgrade relx version

rebar.config

@@ -7,7 +7,7 @@
         {providers,           "1.6.0"},
         {getopt,              "0.8.2"},
         {bbmustache,          "1.0.4"},
-        {relx,                "3.19.0"},
+        {relx,                "3.20.0"},
         {cf,                  "0.2.1"},
         {cth_readable,        "1.2.3"},
         {eunit_formatters,    "0.3.1"}]}.