Fix UnicodeEncodeError of the issue #400

[iurijacob]

When running in windows powershell and pipe to a file an error similar to this occurs: UnicodeEncodeError: 'charmap' codec can't encode characters in position 83-120: character maps to <undefined>

Specifying the encoding resolves the issue.

This error is discussed in https://github.com/jorgebastida/awslogs/issues/400