Add new avialable OTP versions at Travis CI

Sources:
* https://github.com/elixir-lang/elixir/blob/master/.travis.yml
* travis-ci/docs-travis-ci-com#225

Signed-off-by: UENISHI Kota <[email protected]>

Author: kuenishi

.travis.yml

@@ -5,6 +5,10 @@ script: PATH=.:$PATH make check-all
 notifications:
   email: false
 otp_release:
+  - 20.0
+  - 19.3
+  - 19.2
+  - 19.1
   - 19.0
   - 18.3
   - 18.2