fix yahoo.rb: gsub! returns nil on no match

Use gsub instead of gsub! b/c nil is returned when the expression doesn't match:

This works:
> 'no matches'.gsub('"', '\"')
=> "no matches"

This doesn't:
> 'no matches'.gsub!('"', '\"')
=> nil
Merge pull request #1 from kookster/patch-1

fix yahoo.rb: gsub! returns nil on no match