Fix the case when a file from diff does not exist in a container (Mor…

…e details: prontolabs#338)

lib/pronto/runner.rb

@@ -53,7 +53,7 @@ def gem_file?(path)
     end
 
     def ruby_executable?(path)
-      return false if File.directory?(path)
+      return false unless File.file?(path)
       line = File.open(path, &:readline)
       line =~ /#!.*ruby/
     rescue ArgumentError, EOFError