Update rdoc (#1271)

Prior to this commit, we were getting errors when running `bundle exec
rake install`.

Author: stevepolitodesign

Gemfile.lock

@@ -8,8 +8,9 @@ GEM
   specs:
     ast (2.4.2)
     base64 (0.1.2)
-    date (3.4.1)
+    date (3.5.1)
     diff-lcs (1.6.2)
+    erb (6.0.0)
     io-console (0.8.0)
     irb (1.15.1)
       pp (>= 0.6.0)
@@ -25,14 +26,16 @@ GEM
     pp (0.6.2)
       prettyprint
     prettyprint (0.2.0)
-    psych (5.2.3)
+    psych (5.3.0)
       date
       stringio
     racc (1.8.1)
     rainbow (3.1.1)
     rake (13.2.1)
-    rdoc (6.12.0)
+    rdoc (6.17.0)
+      erb
       psych (>= 4.0.0)
+      tsort
     regexp_parser (2.8.2)
     reline (0.6.0)
       io-console (~> 0.5)
@@ -80,7 +83,8 @@ GEM
     standard-performance (1.2.1)
       lint_roller (~> 1.1)
       rubocop-performance (~> 1.19.1)
-    stringio (3.1.3)
+    stringio (3.1.9)
+    tsort (0.2.0)
     unicode-display_width (2.5.0)
 
 PLATFORMS