Update ruby-spreadsheet to 1.0.7.

### 1.0.7 / 23.09.2015

Author: Leopoldo Lee Agdeppa III <leopoldo.agdeppa@gmail.com>
Date:  Wed Sep 23 08:24:16 2015 +0800

* Update worksheet.rb
* Adding Test for Freeze panels
* Update worksheet.rb
* Added freeze (freeze panel) functionality
* Update worksheet.rb
* Freeze (freeze window) functionality added to worksheet

### 1.0.6 / 14.09.2015

Author: Yann Plancqueel <yplancqueel@gmail.com>
Date:  Sat Sep 12 15:32:49 2015 +0200

(taca)