Log Message:
Update ruby-tzinfo to 1.2.3.

v1.2.3							2017/3/25

* Reduce the number of String objects allocated when loading zoneinfo
  files. #54.
* Make Timezone#friendly_identifier compatible with frozen string literals.
* Improve the algorithm for deriving the utc_offset from zoneinfo files. This
  now correctly handles Pacific/Apia switching from one side of the
  International Date Line to the other whilst observing daylight savings
  time. #66.
* Fix an UnknownTimezone exception when calling transitions_up_to or
  offsets_up_to on a TimezoneProxy instance obtained from Timezone.get_proxy.
* Allow the Factory zone to be obtained from the Zoneinfo data source.
* Ignore the /usr/share/zoneinfo/timeconfig symlink included in Slackware
  distributions. #64.
* Fix Timezone#strftime handling of %Z expansion when %Z is prefixed with more
  than one percent. #31.
* Support expansion of %z, %:z, %::z and %:::z to the UTC offset of the time
  zone in Timezone#strftime. #31 and #67.