Log Message:
math/ruby-spreadsheet: update to 1.1.6

### 1.1.6 / 12.03.2018

Author: Todd Hambley <thambley@travelleaders.com>
Date:   Mon Mar 12 14:20:39 2018 -0400

* fix reject for ruby 1.8.7
* fix using invalid code pages when writing workbook

### 1.1.5 / 20.11.2017

Author: Paco Guzmán <pacoguzman@users.noreply.github.com>
Date:   Sun Nov 19 18:10:57 2017 +0100

* Avoid creating a class variable, that variable cannot be garbage collected and \ 
it retains a lot of memory