Revert "Pin yard version to v0.9.28 to avoid ArgumentError"

This reverts commit 12f1eef.

The problem I reported in lsegal/yard#1490 was fixed in this commit [1]
and released in v0.9.31. That means we no longer need to pin the version
of yard.

Fixes #609.

[1]: lsegal/yard@7d05c7f

Gemfile

@@ -26,5 +26,5 @@ if RUBY_VERSION >= '2.2.0'
 end
 if ENV['MOCHA_GENERATE_DOCS']
   gem 'redcarpet'
-  gem 'yard', '<= 0.9.28'
+  gem 'yard'
 end