Set ruby-2.3.8 to allow_failures because of ffi unrelated error

The error occures while:
  rvm use 2.3.8 --install --binary --fuzzy

Text is:
  Libraries missing for ruby-2.3.8: /usr/local/opt/openssl/lib/libcrypto.1.0.0.dylib,/usr/local/opt/openssl/lib/libssl.1.0.0.dylib. Refer to your system manual for installing libraries
  Mounting remote ruby failed with status 10, stopping installation.

See: https://travis-ci.org/ffi/ffi/builds/634215671

.travis.yml

@@ -23,6 +23,8 @@ matrix:
   allow_failures:
     - os: osx
       rvm: ruby-head
+    - os: osx
+      rvm: 2.3.8
   include:
     - name: powerpc
       language: generic