Make binding.eval Ruby 3.0 compatible.

Ruby 3.0 changes behavior of `binding.eval` [[1]] resulting in test
failure:

~~~
WebConsole::EvaluatorTest#test_Evaluator_callers_are_cleaned_up_of_unneeded_backtraces [/home/travis/build/rails/web-console/test/web_console/evaluator_test.rb:63]:
--- expected
+++ actual
@@ -1,3 +1,3 @@
 "RuntimeError: oops
-\tfrom /home/travis/build/rails/web-console/test/web_console/evaluator_test.rb:61:in `block in <class:EvaluatorTest>'
+\tfrom (eval):1:in `block in <class:EvaluatorTest>'
~~~

Fixes #301

[1]: https://bugs.ruby-lang.org/issues/17419

lib/web_console/evaluator.rb

@@ -19,7 +19,14 @@ def initialize(binding = TOPLEVEL_BINDING)
     end
 
     def eval(input)
-      "=> #{@binding.eval(input).inspect}\n"
+      # Binding#source_location is available since Ruby 2.6. But if this should
+      # be removed the `WebConsole::SessionTest#test_use_first_binding_if_no_application_bindings`
+      # must be adjusted.
+      if @binding.respond_to? :source_location
+        "=> #{@binding.eval(input, *@binding.source_location).inspect}\n"
+      else
+        "=> #{@binding.eval(input).inspect}\n"
+      end
     rescue Exception => exc
       format_exception(exc)
     end