Feature/phpunit fix

[JaroslawZielinski]

Upgrade phpunit dependency #11

[JaroslawZielinski]

Hi,
after ygalescot user I bump up the issue:

Thank you for having created this lib :)

Would it be possible to update the phpunit requirement to the "require-dev" please, because it blocks the update of phpunit on other projects.

For instance I would like to update phpunit to version 8 but I can't because of this error:

Your requirements could not be resolved to an installable set of packages.
  Problem 1
    - andrejsstepanovs/business-days-calculator 1.0.0 requires phpunit/phpunit 4.*

Thank you