Log Message:
Update ruby-gettext to 3.1.2.
(restrict acceptable ruby version from ruby-text).

## 3.1.2: 2014-04-24

### Improvements

  * Travis CI: Enabled Rubyinus again.
    [GitHub#30] [Patch by Masafumi Yokoyama]
  * `GetText::Tools::Task`: Added `msginit_options` to customize `msginit`
    command line.
  * `rmsginit`: Supported `--translator` option.
  * `GetText::Tools::Task`: Changed to not set translator information
    by default. [GitHub#31] [Reported by David Silva]

### Thanks

  * Masafumi Yokoyama
  * David Silva