Removed depricated UTF8 calls and replaced with utf8

[orthros]

Looks like this is a Dart 2.0 issue. The failing Travis message is of the error • Undefined name 'utf8' variety The Dart version on the Travis build server is

Dart VM version: 1.24.3 (Wed Dec 13 16:10:39 2017) on "linux_x64"

But locally on my machine running version 2.0.0-dev.35.0

C:\Dev\Personal\dart-epub>dart --version
Dart VM version: 2.0.0-dev.35.0 (Fri Mar 9 13:09:14 2018 +0100) on "windows_x64"

The dartanalyzer call doesn't fail.

[orthros]

This has been fixed in #17