FIX making too many requests

We may be trigerring too many requests to rubygems.
In which case it won't send us the information we need.
If such a thing happens, we should wait a while before making a new
request.

Details
* FIX making too many requests
* ADD specs

lib/gem_updater/ruby_gems_fetcher.rb

@@ -31,10 +31,17 @@ def source_uri
     # @return [String|nil] uri of source code
     def uri_from_rubygems
       return unless source.remotes.map( &:host ).include?( 'rubygems.org' )
+      tries = 0
 
       response = begin
         JSON.parse( open( "https://rubygems.org/api/v1/gems/#{gem_name}.json" ).read )
-      rescue OpenURI::HTTPError
+      rescue OpenURI::HTTPError => e
+        # We may trigger too many requests, in which case give rubygems a break
+        if e.io.status.include?( '429' )
+          if ( tries += 1 ) < 2
+            sleep 1 and retry
+          end
+        end
       end
 
       if response

spec/gem_updater/ruby_gems_fetcher_spec.rb

@@ -5,6 +5,18 @@
 
   describe '#source_uri' do
     context 'when gem exists on rubygems.org' do
+
+      describe 'making too many requests' do
+        before do
+          allow( subject ).to receive_message_chain( :open ) { raise OpenURI::HTTPError.new( '429', OpenStruct.new( status: [ '429' ] ) ) }
+          subject.source_uri
+        end
+
+        it 'tries again' do
+          expect( subject ).to have_received( :open ).twice
+        end
+      end
+
       context "when 'source_code_uri' is present" do
         before do
           allow( subject ).to receive_message_chain( :open, :read ) { { source_code_uri: 'source_code_uri' }.to_json }