upgrade relx to 3.8.0, fixes bug in upgrade_install escript

rebar.config

@@ -7,7 +7,7 @@
         {providers,           "1.5.0"},
         {getopt,              "0.8.2"},
         {bbmustache,          "1.0.4"},
-        {relx,                "3.7.1"},
+        {relx,                "3.8.0"},
         {cf,                  "0.1.3"}]}.
 
 {escript_name, rebar3}.

rebar.lock

@@ -4,5 +4,5 @@
  {<<"erlware_commons">>,{pkg,<<"erlware_commons">>,<<"0.16.0">>},0},
  {<<"getopt">>,{pkg,<<"getopt">>,<<"0.8.2">>},0},
  {<<"providers">>,{pkg,<<"providers">>,<<"1.5.0">>},0},
- {<<"relx">>,{pkg,<<"relx">>,<<"3.7.1">>},0},
+ {<<"relx">>,{pkg,<<"relx">>,<<"3.8.0">>},0},
  {<<"ssl_verify_hostname">>,{pkg,<<"ssl_verify_hostname">>,<<"1.0.5">>},0}].