Add --upgrade flag to pip install in readme (#335)

Summary:
Make sure to install master over any existing botorch installation.
Pull Request resolved: #335

Reviewed By: lena-kashtelyan

Differential Revision: D18815213

Pulled By: Balandat

fbshipit-source-id: e41c2f00bb983f16695cb012f1428f460cb52354

Author: Balandat

README.md

@@ -85,8 +85,8 @@ If you would like to try our bleeding edge features (and don't mind potentially
 running into the occasional bug here or there), you can install the latest
 master directly from GitHub (this will also require installing the current GPyTorch master):
 ```bash
-pip install git+https://github.com/cornellius-gp/gpytorch.git
-pip install git+https://github.com/pytorch/botorch.git
+pip install --upgrade git+https://github.com/cornellius-gp/gpytorch.git
+pip install --upgrade git+https://github.com/pytorch/botorch.git
 ```
 
 **Manual / Dev install**