remove ruby2.0 support

since something seems to have broken it

Author: jcupitt

.github/workflows/test.yml

@@ -38,7 +38,6 @@ jobs:
       matrix:
         os-version: [ 'ubuntu-20.04' ]
         ruby-version:
-          - '2.0'
           - '2.1'
           - '2.2'
           - '2.3'

lib/vips/image.rb

@@ -463,7 +463,7 @@ def self.new_from_source source, option_string, **opts
       # keep a secret ref to the source object ... the libvips loader will
       # keep a ref to the C source object, but we need the ruby wrapper object
       # to stay alive too
-      result.references << source if result.is_a?(Vips::Image)
+      result.references << source 
 
       result
     end