Add a fallback for syntax_valid? if RubyVM is not defined

[eregon]

• Use Ripper as it is a standard library.
• Another possibility would be to use eval() like this:

  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.

While trying byebug on TruffleRuby, I found this is the first issue. This change seems to fix it.

The next issue is supporting the TracePoint :call event in TruffleRuby: https://github.com/oracle/truffleruby/issues/1672.

[eregon]

The CI failure is unrelated.