Avoid negative #source_indent which blows up in certain cases

Some monkey patched implementations of `String#indent`, such as Active
Support's, do not handle negative values (see issue cucumber#620). This behavior
isn't strictly a bug in Cucumber but a lower bound of 0 in
`Cucumber::Ast::HasSteps#source_indent` would avoid these types of
errors from bubbling up.

lib/cucumber/ast/has_steps.rb

@@ -46,7 +46,7 @@ def backtrace_line(step_name = "#{@keyword}: #{name}", line = self.line)
       end
 
       def source_indent(text_length)
-        max_line_length - text_length
+        [max_line_length - text_length, 0].max
       end
 
       def max_line_length