diff --git a/Gemfile.lock b/Gemfile.lock
index 5f4c063..52d327c 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -73,7 +73,7 @@ GEM
       drb
       i18n (>= 1.6, < 2)
       minitest (>= 5.1)
-      mutex_m (0.2.0)
+      mutex_m
       tzinfo (~> 2.0)
     ansi (1.5.0)
     base32 (0.3.4)