Declare the license of ROMA in gemspec file

I know there is LICENSE file in this repository and the file is distributed with the gem package. 
but in rubygems.org, ROMA looks like no license :feelsgood: 

![roma_rubygems](https://cloud.githubusercontent.com/assets/124871/5560090/1bda421c-8db5-11e4-97f6-11628dd58268.png)