Only add composer alias if different from branch name

Otherwise composer gets confused:

  Problem 1
      - The requested package silverstripe/framework could not be found in an
  y version, there may be a typo in the package name.
    Problem 2
      - Installation request for silverstripe/cms 2.4.x-dev -> satisfiable by
   silverstripe/cms 2.4.x-dev.
      - silverstripe/cms 2.4.x-dev requires silverstripe/framework 2.4.* -> n
  o matching package found.