This can be an issue when TZInfo::TimeZone#current_period is refreshed
due to timezone period transition, but it's not reflected in
ActiveSupport::TimeZone object.

For example, on Sun, 26 Oct 2014 22:00 UTC, Moscow changed its TZ from
MSK +04:00 to MSK +03:00 (-1 hour). If ActiveSupport::TimeZone['Moscow']
happens to be initialized just before the timezone transition, it will
cache its stale utc_offset even after the timezone transition.

This commit removes cache and fixes this issue.

Signed-off-by: Jeremy Daer <jeremydaer@gmail.com>