Replace SimpleCache with Dalli

Updated:-

I wanted to use Apache Benchmark but it doesn't support oauth :sweat_smile:.
I noticed that the project is using New Relic. I'm not a fan of rpm, but It works so I only
changed the code to use it in development. These are the results:

## Before

![bad](https://dl.dropboxusercontent.com/u/75705751/bad.png)

## After

![good](https://dl.dropboxusercontent.com/u/75705751/good.png)

Obviously, caching the GitHub API responses is a performance improvement
but the first hit to the board may end in a timeout. I'm not very confident with
the `Ghee` gem (calls the GitHub API). I'll need to take a look at it.

Reference #3.