test meck with Erlang/OTP 21.0

.travis.yml

@@ -3,6 +3,7 @@ language: erlang
 sudo: false
 os: linux
 otp_release:
+  - 21.0
   - 20.0
   - 19.3
   - 18.3

README.md

@@ -271,7 +271,7 @@ donations][coinbase] are greatly appreciated!
 [hex]: https://hex.pm/packages/meck
 [hex version badge]: https://img.shields.io/hexpm/v/meck.svg?style=flat-square
 [hex license badge]: https://img.shields.io/hexpm/l/meck.svg?style=flat-square
-[erlang version badge]: https://img.shields.io/badge/erlang-R15B03%20to%2020.0-blue.svg?style=flat-square
+[erlang version badge]: https://img.shields.io/badge/erlang-R15B03%20to%2021.0-blue.svg?style=flat-square
 [build tool]: https://img.shields.io/badge/build%20tool-rebar3-orange.svg?style=flat-square
 
 <!-- Links -->