Regenerate corrupted Gemfile.lock.

As of bundler version 1.11, a Gemfile.lock with missing dependencies
is treated as 'corrupted'. Regenerating the Gemfile.lock with
bundler 1.12.2 creates a valid Gemfile.lock.

Gemfile.lock

@@ -1,12 +1,14 @@
 GEM
   remote: http://rubygems.org/
   specs:
+    json (1.8.3)
     metaclass (0.0.4)
-    minitest (5.8.3)
+    minitest (5.8.4)
     mocha (1.1.0)
       metaclass (~> 0.0.1)
-    rake (10.4.2)
-    rdoc (4.2.0)
+    rake (11.1.2)
+    rdoc (4.2.2)
+      json (~> 1.4)
 
 PLATFORMS
   ruby
@@ -18,4 +20,4 @@ DEPENDENCIES
   rdoc
 
 BUNDLED WITH
-   1.10.6
+   1.12.2