Use newer ruby

Based on https://github.com/PowerShell/PowerShell/pull/5065

Signed-off-by: Michal Čihař <michal@cihar.com>

.travis.yml

@@ -64,6 +64,9 @@ matrix:
       language: generic
       env: CI_MODE=test
       before_install:
+        # Default 2.0.0 Ruby is buggy
+        - rvm install ruby-2.3.3
+        - rvm --default use 2.3.3
         - brew tap homebrew/php
         - brew update
         - brew install gettext php70 mariadb