fix handling of message in answers

[dwarfmaster]

With my version of coqtop The Coq Proof Assistant, version 8.6 (March 2017), messages content may have changed place in the xml. I am thus updating it.
Should also fix #54

[teto]

LGTM I've had problems with this as well with coq 8.5.

[dwarfmaster]

poke @trefis

[dadoomer]

Thank you!

[yilongli]

Thanks. This patch saves my day. Any reason not to merge it?

[trefis]

@lucas8 in the code around your change I can see we always access the val field of whatever parse_value returned. We don't anymore with your code. Any reason for that change?

[dwarfmaster]

@trefis I don't see what you're speaking about : inside get_answer, the val field of parse_value isn't used anymore. What may cause the misunderstanding is that the type of parse_value depends on the structure of the XML parsed, which apparently changed between the coq versions.

[trefis]

I misread a call to parse_response as a call to parse_value.

I haven't tested but will trust you that this works, merging.