fix a lint error

Author: jcupitt

spec/connection_spec.rb

@@ -20,7 +20,7 @@
   end
 
   it "can create a source from an area of memory" do
-    str = File.open(simg("wagon.jpg"), "rb").read
+    str = File.binread(simg("wagon.jpg"))
     source = Vips::Source.new_from_memory str
 
     expect(source)