Enforce UTF-8 file encoding for javac

@djadmin had build problems, hopefully this should fix them.

https://groups.google.com/d/msg/ceylon-users/H2Es65dZSks/zXFeOyh64dUJ

@djadmin could you test this? Run
```bash
git remote add lucaswerkmeister 'https://github.com/lucaswerkmeister/ceylon-common.git'
git fetch lucaswerkmeister
git checkout enforceUtf8
ant clean build
```
and tell me if it works