Bump rdoc from 3.12 to 3.12.2

Bumps [rdoc](https://github.com/ruby/rdoc) from 3.12 to 3.12.2.
- [Release notes](https://github.com/ruby/rdoc/releases)
- [Changelog](https://github.com/ruby/rdoc/blob/master/History.rdoc)
- [Commits](ruby/rdoc@v3.12...v3.12.2)

Signed-off-by: dependabot[bot] <support@github.com>

Gemfile.lock

@@ -44,7 +44,7 @@ GEM
     jquery-rails (2.0.1)
       railties (>= 3.2.0, < 5.0)
       thor (~> 0.14)
-    json (1.6.5)
+    json (1.8.6)
     mail (2.4.3)
       i18n (>= 0.4.0)
       mime-types (~> 1.16)
@@ -76,7 +76,7 @@ GEM
       rdoc (~> 3.4)
       thor (~> 0.14.6)
     rake (0.9.2.2)
-    rdoc (3.12)
+    rdoc (3.12.2)
       json (~> 1.4)
     rspec (2.8.0)
       rspec-core (~> 2.8.0)