add OTP 21 to .travis.yml

.travis.yml

@@ -2,6 +2,7 @@ language: erlang
 sudo: false
 otp_release:
 # Test on all the latest releases for the major versions supported as specified by the `require_otp_vsn` in rebar.config
+   - 21.0
    - 20.0
    - 19.3
    - 18.0