bump github-linguist dependency to work around transitive dep issues

github-linguist 2.12.0 depends on an old version of pygments.rb which
depends on an old version of yajl-ruby which does not compile on ruby 2.4

Author: Charlie Somerville

gemfiles/rails_5.gemfile.lock

@@ -58,11 +58,11 @@ GEM
     erubis (2.7.0)
     escape_utils (1.0.1)
     gemoji (2.1.0)
-    github-linguist (2.12.0)
+    github-linguist (2.12.1)
       charlock_holmes (~> 0.7.3)
       escape_utils (~> 1.0.1)
       mime-types (~> 1.19)
-      pygments.rb (~> 0.6.0)
+      pygments.rb (> 0.6.0)
     github-markdown (0.6.9)
     globalid (0.3.6)
       activesupport (>= 4.1.0)
@@ -75,15 +75,14 @@ GEM
     mime-types (1.25.1)
     mini_portile2 (2.1.0)
     minitest (5.9.0)
+    multi_json (1.12.1)
     nio4r (1.2.1)
     nokogiri (1.6.8)
       mini_portile2 (~> 2.1.0)
       pkg-config (~> 1.1.7)
     pkg-config (1.1.7)
-    posix-spawn (0.3.11)
-    pygments.rb (0.6.3)
-      posix-spawn (~> 0.3.6)
-      yajl-ruby (~> 1.2.0)
+    pygments.rb (1.1.1)
+      multi_json (>= 1.0.0)
     rack (2.0.1)
     rack-test (0.6.3)
       rack (>= 1.0)
@@ -128,7 +127,6 @@ GEM
     websocket-driver (0.6.4)
       websocket-extensions (>= 0.1.0)
     websocket-extensions (0.1.2)
-    yajl-ruby (1.2.1)
 
 PLATFORMS
   ruby
@@ -150,4 +148,4 @@ DEPENDENCIES
   sanitize (~> 2.0)
 
 BUNDLED WITH
-   1.12.5
+   1.13.7