Exclude images from being bundled with gem (#1142)

Co-authored-by: m-r-mccormick <m-r-mccormick@users.noreply.github.com>

just-the-docs.gemspec

@@ -16,7 +16,7 @@ Gem::Specification.new do |spec|
     "source_code_uri"   => "https://github.com/just-the-docs/just-the-docs",
   }
 
-  spec.files         = `git ls-files -z`.split("\x0").select { |f| f.match(%r{^(assets|bin|_layouts|_includes|lib|Rakefile|_sass|LICENSE|README|CHANGELOG|favicon)}i) }
+  spec.files         = `git ls-files -z ':!:*.jpg' ':!:*.png'`.split("\x0").select { |f| f.match(%r{^(assets|bin|_layouts|_includes|lib|Rakefile|_sass|LICENSE|README|CHANGELOG|favicon)}i) }
   spec.executables   << 'just-the-docs'
 
   spec.add_development_dependency "bundler", "~> 2.3.5"