Pass encoding to readFileSync as a string to maintain backwards compatibility

html2jade was failing with node v0.8 and earlier due to the encoding being passed to readFileSync as an options object. This commit reverts to passing the encoding as a string to maintain backwards compatibility ([this is still supported in v0.10](https://github.com/joyent/node/wiki/Api-changes-between-v0.8-and-v0.10)).

Prior to this fix, running html2jade with node v0.8 would log a misleading "unknown encoding" error for any html file. I believe that is what was occurring in issue https://github.com/donpark/html2jade/issues/53.