Drop support for LibreSSL 2.3

LibreSSL 2.3 reached its EOL on 2017-03[1]. No code changes are needed.

[1] http://www.libressl.org/releases.html

Author: rhenium

.travis.yml

@@ -21,7 +21,6 @@ matrix:
     - env: RUBY_VERSION=ruby-2.4 OPENSSL_VERSION=openssl-1.0.1
     - env: RUBY_VERSION=ruby-2.4 OPENSSL_VERSION=openssl-1.0.2
     - env: RUBY_VERSION=ruby-2.4 OPENSSL_VERSION=openssl-1.1.0
-    - env: RUBY_VERSION=ruby-2.4 OPENSSL_VERSION=libressl-2.3
     - env: RUBY_VERSION=ruby-2.4 OPENSSL_VERSION=libressl-2.4
     - env: RUBY_VERSION=ruby-2.4 OPENSSL_VERSION=libressl-2.5
     - language: ruby