resolves #1671 install Kindlegen so .mobi output can be produced

slonopotamus · slonopotamus · commit 0a6a5110b82c · 2021-06-29T19:53:45.000+03:00
diff --git a/Gemfile b/Gemfile
@@ -14,3 +14,4 @@ gem 'pygments.rb', '2.2.0'
 gem 'thread_safe', '0.3.6'
 gem 'epubcheck-ruby', '4.2.5.0'
 gem 'html-proofer', '3.19.2'
+gem 'kindlegen', '3.1.1'
