Add OTP 20 and 21 to travis, update proper

* Add OTP 20.3 to travis
* Add OTP 21.1 to travis
* Pin proper to 1.3 to resolve noisy deprecation warnings during test runs

Author: nalundgaard

.travis.yml

@@ -5,7 +5,9 @@ otp_release:
   - R16B03-1
   - 17.5
   - 18.3
-  - 19.0
+  - 19.3
+  - 20.3
+  - 21.1
 branches:
   only:
     - master

rebar.config

@@ -27,7 +27,7 @@
                     {eunit_opts, [{report,{eunit_surefire,[{dir,"."}]}}]}
                    ,{cover_enabled, true}
                    ,{deps, [
-                            {proper, {git, "https://github.com/manopapad/proper.git", {ref, "v1.2"}}}
+                            {proper, {git, "https://github.com/manopapad/proper.git", {ref, "v1.3"}}}
                            ]}
                    ]}
            ]}.