upgrade relx to 3.12.0

rebar.config

@@ -7,7 +7,7 @@
         {providers,           "1.6.0"},
         {getopt,              "0.8.2"},
         {bbmustache,          "1.0.4"},
-        {relx,                "3.11.0"},
+        {relx,                "3.12.0"},
         {cf,                  "0.2.1"},
         {cth_readable,        "1.1.1"},
         {eunit_formatters,    "0.3.1"}]}.

rebar.lock

@@ -6,5 +6,5 @@
  {<<"eunit_formatters">>,{pkg,<<"eunit_formatters">>,<<"0.3.1">>},0},
  {<<"getopt">>,{pkg,<<"getopt">>,<<"0.8.2">>},0},
  {<<"providers">>,{pkg,<<"providers">>,<<"1.6.0">>},0},
- {<<"relx">>,{pkg,<<"relx">>,<<"3.11.0">>},0},
+ {<<"relx">>,{pkg,<<"relx">>,<<"3.12.0">>},0},
  {<<"ssl_verify_hostname">>,{pkg,<<"ssl_verify_hostname">>,<<"1.0.5">>},0}].