Update requirements.txt

requirements.txt

-conda environment with python=3.6, 
-install astra-toolbox stable version
+install conda environment with python=3.6, 
+install astra-toolbox stable version(```conda install -c astra-toolbox astra-toolbox```)
+```
 conda install pytorch torchvision cudatoolkit=9.0 -c pytorch
 conda install dival
+```
 
 Make sure to integrate it to the Jupyter kernel.