Fix gemspec encoding

Hi Joël,

Your name give some conflict if you don't set utf-8 encoding.

```
There was a SyntaxError while loading aureus.gemspec: 
syntax error, unexpected end-of-input, expecting keyword_end
  s.author = 'Joël Gähhwiler'
```
Merge pull request #1 from thibaudgg/patch-1

Fix gemspec encoding