Update .travis.yml

Try Erlang/OTP 20.1 

* older version of rebar3 without the problem with pre-hooks?
  See erlang/rebar3#1804

Author: pmenhart

.travis.yml

@@ -7,11 +7,16 @@ elixir:
 otp_release:
 - 18.3
 - 19.3
+- 20.1
 - 20.3
 matrix:
   exclude:
+  - elixir: 1.3.4
+    otp_release: 20.1
   - elixir: 1.3.4
     otp_release: 20.3
+  - elixir: 1.4.5
+    otp_release: 20.1
   - elixir: 1.4.5
     otp_release: 20.3
   - elixir: 1.6.5