Remove OTP 20 as there is no elixir version built against it for 1.10.4

Author: jsmestad

.travis.yml

@@ -9,8 +9,9 @@ elixir:
   - 1.9.4
   - 1.10.4
 otp_release:
-  - 20.3
+  - 21.3
   - 22.3
+  - 23.0
 before_install:
   - curl -fsSkL https://gist.githubusercontent.com/Trevoke/90409590f43abfd0d7fe3b02b5249849/raw > travis.sh && source ./travis.sh
   - evm install emacs-$EVM_EMACS-travis-linux-xenial --use --skip