Run 21.3.8 on Travis due to regression on 21.3.0 (#9088)

Author: José Valim

.travis.yml

@@ -6,7 +6,7 @@ env:
     - ELIXIR_ASSERT_TIMEOUT=2000
   matrix:
     - OTP_RELEASE=OTP-22.0 CHECK_REPRODUCIBLE=true CHECK_POSIX_COMPLIANT=true
-    - OTP_RELEASE=OTP-21.3
+    - OTP_RELEASE=OTP-21.3.8
     - OTP_RELEASE=OTP-21.2
     - OTP_RELEASE=OTP-21.1
     - OTP_RELEASE=OTP-21.0