Update .travis.yml

increase minimum otp version to 20.3.8

.travis.yml

@@ -1,6 +1,6 @@
 language: erlang
 otp_release:
-  - 19.3
+  - 20.3.8
 script:
   - "curl -fsSL https://raw.github.com/synrc/mad/master/mad > mad && chmod +x mad && ./mad dep com"
   - "rebar3 dialyzer"