Travis CI build broken by IANA 2017c

[Hugo]

The first broken build was the one which added IANA 2017c:

https://travis-ci.org/stub42/pytz/builds/294881002#L2684

```
cc -DTZDIR=\"/home/travis/build/stub42/pytz/build/etc/zoneinfo\" -c -o zdump.o zdump.c
zdump.c:803:1: error: static declaration of ‘snprintf’ follows non-static declaration
 snprintf(char *s, size_t size, char const *format, ...)
 ^
```

[Stuart Bishop (stub)]

This should be fixed the next time the IANA code is synced into pytz