mruby-time: remove unnecessary `tzset` call

The gem doesn't use the global variables and:
>Local timezone information is used as though localtime() calls tzset().
http://pubs.opengroup.org/onlinepubs/9699919799/functions/localtime.html
mruby-time: fix check of  `mktime`'s return value

>If the calendar time cannot be represented, the function returns the
value (time_t)(-1).
http://www.iso-9899.info/n1256.html#7.23.2.3p3
Merge pull request #2058 from cremno/mruby-time-more-portable

mruby-time: two small changes for portability