Bug#811635: FTBFS with GCC 6: cannot convert x to y

[peter green]

tags 811635 +patch
thanks

 > document.cxx:250:12: error: cannot convert 'std::ostream {aka 
std::basic_ostream<char>}' to 'bool' in return
 > return os;
 > ^~

Adding an explicit cast to bool fixes this.

I have uploaded a fix for this to raspbian. The dsc is at 
http://plugwash.raspbian.org/jessietest/private/pool/main/libf/libfolia/libfolia_0.13-2+rpi1.dsc 
. A debdiff should turn up soon at 
https://debdiffs.raspbian.org/main/libf/libfolia/libfolia_0.13-2+rpi1.debdiff