Adds composer.json for Thrift dev-master branch

Adds composer.json for Thrift dev-master branch

Given that the next release will likely be 1.0.0, i've aliased the dev-master branch to 1.0.x-dev, so anyone requiring 1.0.x-dev will get the most recent commit in dev-master.

When 1.0.0 will be tagged/released, supposedly creating a branch for it (e.g. 1.0.x), we'll switch (on master) the alias to 1.1.x-dev, so:

- anyone requiring 1.0.* will get the most recent tagged version (stable release) along branch 1.0.x (until lowering minimum-stability to dev, in which case it will yield the most recent commit along that branch)
- anyone requiring 1.0.x-dev will get the most recent commit in branch 1.0
- anyone requiring 1.1.x-dev will get the most recent commit in dev-master.

And so on