Revert "Prefer distributions with Composer"
Revert "Prefer distributions with Composer"

This reverts commit eb34a294bdbc35fb0bda7c71d50a356a13225357.

Props @staabm for pointing out `--prefer-dist` is API limited, which can cause
problems on Travis.

meox/Requests,andreipetcu/Requests,dskanth/Requests,mubassirhayat/Requests,meox/Requests,meox/Requests