Change gem's homepage uri to GitHub due to RubyForge's sunset

Hi, I found that homepage URL of diff-lcs gem is pointing to RubyForge.
But RubyForge will be shutdown at May 15th ([announce](https://twitter.com/evanphx/status/399552820380053505)), the URL http://diff-lcs.rubyforge.org/ will be gone.

This pull request changes homepage URI definition of gemspec to GitHub project page.