Set default encoding to utf-8 in vimrc

Fixes https://github.com/chrishunt/dot-files/issues/6
Thanks @dongbeta!

From the vim documentation:


> NOTE: For MacVim and GTK+ 2 it is highly recommended to set 'encoding' to "utf-8".  Although care has been taken to allow different values of 'encoding', "utf-8" is the natural choice for the environment and avoids unnecessary conversion overhead.
>
> **"utf-8" has not been made the default to prevent different behavior of the GUI and terminal versions, and to avoid changing the encoding of newly created files without your knowledge** (in case 'fileencodings' is empty).