Add a fallback for syntax_valid? if RubyVM is not defined (#727)
Add a fallback for syntax_valid? if RubyVM is not defined (#727)

* Use Ripper as it is a standard library.
* Another possibility would be to use eval() like this:
  ```ruby
  catch(:valid) do
    eval("BEGIN{throw :valid, true}\n#{code}")
    false
  end
  ```
  as done in https://github.com/banister/method_source/blob/836d7047ab/lib/method_source/code_helpers.rb#L66-L80
  But that means creating AST nodes, etc and seems heavier.
deivid-rodriguez/byebug,deivid-rodriguez/byebug,deivid-rodriguez/byebug