Nokogiri 1.7 removes support for Rubies < 2.1

See
sparklemotion/nokogiri@8487038

Author: bf4

Gemfile

@@ -28,6 +28,10 @@ if RUBY_VERSION < '2'
   gem 'mime-types', [ '>= 2.6.2', '< 3' ]
 end
 
+if RUBY_VERSION < '2.1'
+  gem 'nokogiri', '< 1.7'
+end
+
 # https://github.com/bundler/bundler/blob/89a8778c19269561926cea172acdcda241d26d23/lib/bundler/dependency.rb#L30-L54
 @windows_platforms = [:mswin, :mingw, :x64_mingw]