Limit supported Erlang versions to 20 and 21

.travis.yml

@@ -2,8 +2,8 @@ language: erlang
 otp_release:
   - 21.0
   - 20.3
-  - 19.3
-  - 18.3
+  #- 19.3 not supported (string:lexemes/2)
+  #- 18.3 not supported (string:lexemes/2, tricky binary comprehension, map typespec with `:=`, ?assertEqual/3)
 script:
   - ./rebar3 compile
   - ./rebar3 eunit

README.md

@@ -22,6 +22,10 @@ Install deps (ubuntu 18.04)
 sudo apt install erlang-nox erlang-dev build-essential
 ```
 
+You need Erlang version 20 or higher! If your version is older, please, check
+[Erlang solutions packages](https://www.erlang-solutions.com/resources/download.html)
+or use [kerl](https://github.com/kerl/kerl).
+
 Get the code:
 
 ```