removed SilverStripe requirements

Composer was giving me this:
  Problem 1
    - Installation request for priithansen/silverstripe-markdowntextareafield dev-master -> satisfiable by priithansen/silverstripe-markdowntextareafield[dev-master].
    - Conclusion: remove silverstripe/cms 3.1.2
    - priithansen/silverstripe-markdowntextareafield dev-master requires silverstripe/cms >=3.1.x-dev,<4.0 -> satisfiable by silverstripe/cms[3.1.x-dev, 3.2.x-dev].
    - Can only install one of: silverstripe/cms[3.1.x-dev, 3.1.2].
    - Can only install one of: silverstripe/cms[3.2.x-dev, 3.1.2].
    - Installation request for silverstripe/cms 3.1.2 -> satisfiable by silverstripe/cms[3.1.2].