Update CI to only test on OTP-20+

Makefile

@@ -29,10 +29,10 @@ dep_gun = git https://github.com/ninenines/gun 1.2.0
 dep_ci.erlang.mk = git https://github.com/ninenines/ci.erlang.mk master
 DEP_EARLY_PLUGINS = ci.erlang.mk
 
-AUTO_CI_OTP ?= OTP-19+
+AUTO_CI_OTP ?= OTP-20+
 AUTO_CI_HIPE ?= OTP-LATEST
 # AUTO_CI_ERLLVM ?= OTP-LATEST
-AUTO_CI_WINDOWS ?= OTP-19+
+AUTO_CI_WINDOWS ?= OTP-20+
 
 # Standard targets.