Update ruby-gettext to3.1.7.

## 3.1.7: 2015-09-22 {#version-3-1-7}

### Improvements

  * Supported 3 character language names.
    [GitHub#39][Patch by Vilius Paulauskas]
  * Suppressed duplicated range in regular expression warning.
    [GitHub#40][Patch by Vilius Paulauskas]
  * Replaced invalid character instead of raising an error on encoding
    conversion.
    [GitHub#41][Patch by Vilius Paulauskas]

### Fixes

  * Fixed a bug that undefined method is used.
    [GitHub#38][Patch by Hiroshi Hatake]

### Thanks

  * Hiroshi Hatake
  * Vilius Paulauskas

(taca)