Bump concurrent-ruby and concurrent-ruby-ext to 1.1.5

This matches the version used in Gitaly and avoids potential
version conflicts when multiple versions are installed.

Gemfile.lock

@@ -139,9 +139,9 @@ GEM
     concord (0.1.5)
       adamantium (~> 0.2.0)
       equalizer (~> 0.0.9)
-    concurrent-ruby (1.1.3)
-    concurrent-ruby-ext (1.1.3)
-      concurrent-ruby (= 1.1.3)
+    concurrent-ruby (1.1.5)
+    concurrent-ruby-ext (1.1.5)
+      concurrent-ruby (= 1.1.5)
     connection_pool (2.2.2)
     crack (0.4.3)
       safe_yaml (~> 1.0.0)