Travis CI: Use -DXML_POOR_ENTROPY to fix compilation
Travis CI: Use -DXML_POOR_ENTROPY to fix compilation

I would use libbsd but it's broken on Ubuntu trusty:
```
/usr/include/bsd/stdlib.h:52:27: error: unknown type name 'u_char'
 void arc4random_addrandom(u_char *dat, int datlen);
                           ^
```
https://travis-ci.org/libexpat/libexpat/jobs/252880355

libexpat/libexpat,libexpat/libexpat,libexpat/libexpat,libexpat/libexpat,libexpat/libexpat,libexpat/libexpat