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

.travis.yml

@@ -6,6 +6,7 @@ dist: trusty
 
 env:
   global:
+    - CPPFLAGS='-DXML_POOR_ENTROPY'  # bad idea, do not copy!
     - CFLAGS='-g -pipe'
   matrix:
     - MODE=address