Remove Yii2 source code from this repository

This removes the source code of the yii2 framework from this repository, and relies on composer to download the source code during the install stage.
 
benefits: 
* using composer allows us to easily track the version of yii2 being tested (currently you have to look into the source code to find the version)
* removes hundreds of *.php files that were not needed

This has [already been tested](https://travis-ci.org/hamiltont/FrameworkBenchmarks/jobs/44593706) using my personal travis and vagrant and passed all checks