add jekyll-seo-tag to dependency to fix bundle crash

Author: gebeto

_config.yml

@@ -35,3 +35,6 @@ color_scheme: nil
 # Google Analytics Tracking (optional)
 # e.g, UA-1234567-89
 ga_tracking: UA-2709176-10
+
+plugins:
+  - jekyll-seo-tag

just-the-docs.gemspec

@@ -14,6 +14,7 @@ Gem::Specification.new do |spec|
   spec.executables   << 'just-the-docs'
 
   spec.add_runtime_dependency "jekyll", "~> 3.8.5"
+  spec.add_runtime_dependency "jekyll-seo-tag", "~> 2.0"
   spec.add_runtime_dependency "rake", "~> 12.3.1"
 
   spec.add_development_dependency "bundler", "~> 2.0.1"