fixed Bitbucket's broken API

This PR changes the way the ```composer.json``` is fetched. This endpoint always returns JSON which eventually contains a ```data``` key containing the actual file content -- if it was found. Otherwise the response contains the file and directory list.

* JSON for [no composer.json found](https://bitbucket.org/api/1.0/repositories/SallyCMS/sallycms-core/src/253567e4262791753d848713960b6db613526eda/composer.json)
* JSON for [file found](https://bitbucket.org/api/1.0/repositories/SallyCMS/sallycms-core/src/tip/composer.json)

This is basically untested, as I have no quick idea on how to test this out.