Write test to make sure licenses and header comments have correct year

[jonrohan]

I would like the build to fail if the current year doesn't match the year in the LICENSE files. So we'll know to update when the year changes.

Same goes for the header files in the modules.

https://github.com/primer/primer-css/blob/master/modules/primer-css/index.scss#L1-L6

[agisilaos]

Love the idea @jonrohan!
For now, do we need to change this part

https://github.com/primer/primer-css/blob/3190c9cbae211b2ba04b15851245e79328002d60/modules/primer-css/index.scss#L1-L6

to this part:

Released under MIT license. Copyright 2017 GitHub, Inc. ?

[jonrohan]

I'll fix them up in the test PR, good test material.

[jonrohan]

Fixed in #384