Update version to 2017.4.0f1
Update version to 2017.4.0f1

This Unity version has long term support and has a critical
bug fix which the Toolkit hits.

Change-Id: Ifcc393d40ad2c40dbc7e48557b7cf3a35b9bedf7

googlevr/tilt-brush-toolkit,googlevr/tilt-brush-toolkit