Re-license Composer Default plugin under MIT

... from GPL-3.0 license.

Sign-offs have been retrieved from the following individuals:

  * @julianlam, @barisusakli, @psychobunny (as part of @NodeBB)
  * @BenLubar
  * @RaceProUK
  * @pichalite
  * @pitaj
  * @arasbm
  * @frissdiegurke
  * @yariplus

A digital and/or hard copy of the sign-offs can be requested via:

  * email: [email protected]
  * in-person/mail: 619-341 King St. East, Toronto, ON, M5A 1L1

Author: julianlam