Test installation of pip --pre (#7314)
Test installation of pip --pre (#7314)

* Enable test for pip-mkl

* Add --pre flag to test pre-releases

* Fix conflicts

Guneet-Dhillon/mxnet,Guneet-Dhillon/mxnet,Guneet-Dhillon/mxnet,Guneet-Dhillon/mxnet,Guneet-Dhillon/mxnet,Guneet-Dhillon/mxnet,Guneet-Dhillon/mxnet,Guneet-Dhillon/mxnet,Guneet-Dhillon/mxnet