Add a license field to package.json

[miketaylr]

npm WARN package.json webcompat@ No license field.

We use MPL 2: https://github.com/webcompat/webcompat.com/#license

[deepthivenkat]

r? @miketaylr

[karlcow]

@deepthivenkat It's usually better if you ask for review on the Pull Request itself.
#1019

[miketaylr]

Yep, just ping me for review on the PR when it's ready. 👍

[deepthivenkat]

#1038