Update charlock_holmes

The compiler needs to be pinned to c++11, which is done by
https://github.com/brianmario/charlock_holmes/commit/51d8a2f0eb1e9d3e476482727d4122b3d11b2bc6

Without this patch it won't install charlock_holmes on newer compilers.

Gemfile.lock

@@ -113,7 +113,7 @@ GEM
       activesupport (>= 4.0.0)
       mime-types (>= 1.16)
     cause (0.1)
-    charlock_holmes (0.7.3)
+    charlock_holmes (0.7.4)
     chronic (0.10.2)
     chronic_duration (0.10.6)
       numerizer (~> 0.1.1)