Logger OTP-21-rc2 support

Also small output fix in rebar3 shell

rebar.config

@@ -9,7 +9,7 @@
         {bbmustache,          "1.3.0"},
         {relx,                "3.24.4"},
         {cf,                  "0.2.2"},
-        {cth_readable,        "1.4.0"},
+        {cth_readable,        "1.4.1"},
         {eunit_formatters,    "0.5.0"}]}.
 
 {post_hooks, [{"(linux|darwin|solaris|freebsd|netbsd|openbsd)",

rebar.lock

@@ -2,7 +2,7 @@
 [{<<"bbmustache">>,{pkg,<<"bbmustache">>,<<"1.3.0">>},0},
  {<<"certifi">>,{pkg,<<"certifi">>,<<"2.0.0">>},0},
  {<<"cf">>,{pkg,<<"cf">>,<<"0.2.2">>},0},
- {<<"cth_readable">>,{pkg,<<"cth_readable">>,<<"1.4.0">>},0},
+ {<<"cth_readable">>,{pkg,<<"cth_readable">>,<<"1.4.1">>},0},
  {<<"erlware_commons">>,{pkg,<<"erlware_commons">>,<<"1.1.0">>},0},
  {<<"eunit_formatters">>,{pkg,<<"eunit_formatters">>,<<"0.5.0">>},0},
  {<<"getopt">>,{pkg,<<"getopt">>,<<"1.0.1">>},0},
@@ -14,7 +14,7 @@
  {<<"bbmustache">>, <<"2010ADAE78830992A4C69680115ECD7D475DD03A72C076BBADDCCBF2D4B32035">>},
  {<<"certifi">>, <<"A0C0E475107135F76B8C1D5BC7EFB33CD3815CB3CF3DEA7AEFDD174DABEAD064">>},
  {<<"cf">>, <<"7F2913FFF90ABCABD0F489896CFEB0B0674F6C8DF6C10B17A83175448029896C">>},
- {<<"cth_readable">>, <<"F51362E3D365B14DE5AED7E98166E0643F38D004AF9750369D8F90E9F470FCC7">>},
+ {<<"cth_readable">>, <<"1A54C8C73E80D86FDC91EFDC9A5D342335E426C9F26441ACAA59D54DB80E05C7">>},
  {<<"erlware_commons">>, <<"F69F3D96044C2A9E735CCD76F469FEC5FC851797E5FE23115698B4EDC072191B">>},
  {<<"eunit_formatters">>, <<"6A9133943D36A465D804C1C5B6E6839030434B8879C5600D7DDB5B3BAD4CCB59">>},
  {<<"getopt">>, <<"C73A9FA687B217F2FF79F68A3B637711BB1936E712B521D8CE466B29CBF7808A">>},

src/rebar_prv_shell.erl

@@ -236,7 +236,7 @@ maybe_run_script(State) ->
             ?DEBUG("No script_file specified.", []),
             ok;
         "none" ->
-            ?DEBUG("Shell script execution skipped (--script none).", []),
+            ?DEBUG("Shell script execution skipped (--script_file none).", []),
             ok;
         RelFile ->
             File = filename:absname(RelFile),