[DRE-maint] Bug#815770: ruby-mysql2: FTBFS against ruby2.3

[terceiro at debian.org]

Package: ruby-mysql2
Version: 0.3.18-1
Severity: important

During a rebuild with ruby2.3 enabled, ruby-mysql2 fails to build. Please make
sure it builds against ruby2.3, which will be the Ruby version in stretch.

The full build log can be found at
https://pkg-ruby-extras.alioth.debian.org/rebuilds/ruby2.3-build-logs/ruby-mysql2_0.3.18-1_amd64.txt