Log Message:
Update ruby-gettext to 3.1.6.

## 3.1.6: 2015-01-20 {#version-3-1-6}

### Fixes

  * Added a missing required text gem version.
    [GitHub#37][Reported by Hans de Graaff]

### Thanks

  * Hans de Graaff

## 3.1.5: 2015-01-17 {#version-3-1-5}

### Improvements

  * `rmsgmerge`: Improves fuzzy matching speed.
    [GitHub#36][Reported by Dominic Cleal]

### Fixes

  * Fixed a bug that msgid uses wrong encoding.
    [Patch by OBATA Akio]
  * Fixed a typo in document.
    [GitHub#35][Patch by Masafumi Yokoyama]
  * `rmsgmerge`: Fixed a bug that `fuzzy` flag may be added twice.

### Thanks

  * OBATA Akio
  * Masafumi Yokoyama
  * Dominic Cleal

## 3.1.4: 2014-08-26 {#version-3-1-4}

### Improvements

  * Improved document markups by Markdown.
    [GitHub#33] [Patch by Masafumi Yokoyama]

### Fixes

  * `GetText::Tools::Task`: Fixed a bug that `Errno::ENOENT: No such
    file or directory @ rb_file_s_mtime - doc/po/ja/rroonga.edit.po`
    like error is occurred.
  * `GetText::Tools::Task`: Fixed markup in document.

### Thanks

  * Masafumi Yokoyama