Lock concurrent-ruby to 1.3.4

1.3.5 introduced a change that broke activesupport
(and so middleman, which uses it). It might be to
do with it dropping Logger as a dependency but
this is unclear. Full release notes are:

https://github.com/ruby-concurrency/concurrent-ruby/releases/tag/v1.3.5

The activesupport gem fixed this in v7-0-stable of
Rails:

rails/rails#54264

So we're waiting for middleman to pull that
version in. They have an issue for this:

middleman/middleman#2788

govuk_tech_docs.gemspec

@@ -39,6 +39,7 @@ Gem::Specification.new do |spec|
   spec.add_dependency "base64" # TODO: remove once middleman-sprockets declares this itself.
   spec.add_dependency "bigdecimal" # TODO: remove once activesupport declares this itself.
   spec.add_dependency "chronic", "~> 0.10.2"
+  spec.add_dependency "concurrent-ruby", "1.3.4" # 1.3.5 introduced a change that breaks activesupport, and so middleman
   spec.add_dependency "csv" # TODO: remove once tilt declares this itself.
   spec.add_dependency "haml", "~> 6.0"
   spec.add_dependency "middleman", "~> 4.0"