Add meta utf-8 HTML tag in debugger.eex template

Simple fix to enable utf8 encoding in the debugger page.

**From this**:
![image](https://cloud.githubusercontent.com/assets/464900/5534193/9ca68d96-8a28-11e4-84dd-11af420d34c4.png)

**To this**:
![image](https://cloud.githubusercontent.com/assets/464900/5534188/88bf8b98-8a28-11e4-9454-6bff1177b8e1.png)