Add Ruby 2.6.1 to prebuilt list (#164)

Author: dazuma

ruby-pipeline/ruby-latest.yaml

@@ -43,6 +43,7 @@ steps:
            '--prebuilt-image=2.5.1=gcr.io/gcp-runtimes/ruby/ubuntu16/prebuilt/ruby-2.5.1:latest',
            '--prebuilt-image=2.5.3=gcr.io/gcp-runtimes/ruby/ubuntu16/prebuilt/ruby-2.5.3:latest',
            '--prebuilt-image=2.6.0=gcr.io/gcp-runtimes/ruby/ubuntu16/prebuilt/ruby-2.6.0:latest',
+           '--prebuilt-image=2.6.1=gcr.io/gcp-runtimes/ruby/ubuntu16/prebuilt/ruby-2.6.1:latest',
            '--default-ruby-version=2.3.8']
   - name: 'gcr.io/kaniko-project/executor:v0.8.0'
     args: ['--destination=$_OUTPUT_IMAGE']