refactor: apply Rubocop to Ruby generator (google#956)

Cherry-pick of G-Rath/osv-detector#240

---

This is all pretty harmless but there's some nice small improvements in
here

scripts/generators/generate-rubygems-versions.rb

@@ -13,7 +13,7 @@
 # such comparisons in the detector would in fact be wrong.
 #
 # @type [Array<String>]
-UNSUPPORTED_COMPARISONS = []
+UNSUPPORTED_COMPARISONS = [].freeze
 
 # @param [String] line
 # @return [Boolean]
@@ -24,12 +24,12 @@ def is_unsupported_comparison?(line)
 # @param [String] line
 # @return [String]
 def uncomment(line)
-  line.sub(/^#|\/\//, "")
+  line.sub(%r{^#|//}, "")
 end
 
 def download_rubygems_db
   URI.open("https://osv-vulnerabilities.storage.googleapis.com/RubyGems/all.zip") do |zip|
-    File.open("rubygems-db.zip", "wb") { |f| f.write(zip.read) }
+    File.binwrite("rubygems-db.zip", zip.read)
   end
 end
 
@@ -38,6 +38,8 @@ def extract_packages_with_versions(osvs)
 
   osvs.each do |osv|
     osv["affected"].each do |affected|
+      next unless affected["package"]["ecosystem"] == "RubyGems"
+
       package = affected["package"]["name"]
 
       packages[package] ||= []
@@ -47,7 +49,7 @@ def extract_packages_with_versions(osvs)
     end
   end
 
-  packages.map { |k, v| [k, v.uniq.sort] }.to_h
+  packages.transform_values { |v| v.uniq.sort }
 end
 
 def compare_version(v1, op, v2)
@@ -72,7 +74,7 @@ def compare_versions(lines, select = :all)
       next
     end
 
-    parts = line.split(" ")
+    parts = line.split
     v1 = parts[0]
     op = parts[1]
     v2 = parts[2]
@@ -117,15 +119,15 @@ def generate_version_compares(versions)
 def generate_package_compares(packages)
   comparisons = []
 
-  packages.each { |_, versions| comparisons.concat(generate_version_compares(versions)) }
+  packages.each_value { |versions| comparisons.concat(generate_version_compares(versions)) }
 
   comparisons
 end
 
 def fetch_packages_versions
   download_rubygems_db
 
-  osvs = Zip::File.open("rubygems-db.zip").map { |f| JSON.load(f.get_input_stream.read) }
+  osvs = Zip::File.open("rubygems-db.zip").map { |f| JSON.parse(f.get_input_stream.read) }
 
   extract_packages_with_versions(osvs)
 end
@@ -134,7 +136,7 @@ def fetch_packages_versions
 
 packs = fetch_packages_versions
 
-File.open(outfile, "w") { |f| f.write(generate_package_compares(packs).uniq.join("\n") + "\n") }
+File.write(outfile, "#{generate_package_compares(packs).uniq.join("\n")}\n")
 
 # set this to either "failures" or "successes" to only have those comparison results
 # printed; setting it to anything else will have all comparison results printed