Add a unittest to check the license.md file date range

The end year in the license file should be "this year" and this should be checked on every run of the tests. If the end year is no correct, fail the test.
example: 
> Copyright (c) 2012-2016 SendGrid, Inc.
It is 2017, so this should be:
> Copyright (c) 2012-2017 SendGrid, Inc.
and the  test should fail.
--done 
