gnu: Add ruby-concurrent.

* gnu/packages/ruby.scm (ruby-concurrent): New variable.

gnu/packages/ruby.scm

@@ -193,6 +193,25 @@ a focus on simplicity and productivity.")
                (("/bin/sh") (which "sh")))
              #t)))))))
 
+(define-public ruby-concurrent
+  (package
+    (name "ruby-concurrent")
+    (version "1.0.5")
+    (source (origin
+              (method url-fetch)
+              (uri (rubygems-uri "concurrent-ruby" version))
+              (sha256
+               (base32
+                "183lszf5gx84kcpb779v6a2y0mx9sssy8dgppng1z9a505nj1qcf"))))
+    (build-system ruby-build-system)
+    (arguments `(#:tests? #f)); No rakefile
+    (home-page "https://github.com/ruby-concurrency/concurrent-ruby")
+    (synopsis "Concurrency tools for Ruby")
+    (description "This gem provides concurrency tools for Ruby.  It provides
+a library of common thread-safe types and data-structures as well as abstractions
+for concurrency and communication between threads.")
+    (license license:expat)))
+
 (define-public ruby-highline
   (package
     (name "ruby-highline")