Add back older versions of Erlang in Travis.

.travis.yml

@@ -4,6 +4,10 @@ otp_release:
   - 22.1
   - 21.3
   - 20.1
+  - 19.3
+  - 18.3
+  - 17.5
+  - R16B03-1
 
 branches:
   only: