Check minimum Ruby version

[ronald]

.gemspec specifies:

s.required_ruby_version = ">= 1.9"

but https://github.com/manet-marketing/gelf_redux/blob/43d1469fbb9bfd608041319c423002a44b1f9e6c/lib/gelf/transport/https.rb#L6 uses keyword arguments which were introduced with Ruby 2.0

Todo:

• check/test minimum Ruby version

[ronald]

v4.0 => Ruby >= 2.6