Set env variable FORCE_COLOR.

[coreyfarrell]

This ensures that chalk uses always color instead of detecting from the
environment. I believe when travis-ci runs the test it chalk detects
that color is not support.

The failure can be reproduced locally (without this patch) by running:

# Fails
env -i npm test --scripts-prepend-node-path

# Passes due to TERM environment
env -i TERM=xterm npm test --scripts-prepend-node-path

I set FORCE_COLOR as it's documented by chalk as how to disable detection.

[bcoe]

@coreyfarrell great find 👍