Bump BoTorch version prior to release (facebook#1071)

Summary:
Pull Request resolved: facebook#1071

Current Ax version is using botorch 0.6.4, causing errors. Bumping required version of botorch to the current botorc release. An Ax release is to be made after this commit.

Reviewed By: saitcakmak

Differential Revision: D38711411

fbshipit-source-id: d5a49400fc340bc4b88519676d78f5557720702f

setup.py

@@ -9,7 +9,7 @@
 from setuptools import find_packages, setup
 
 # TODO: read pinned Botorch version from a shared source
-PINNED_BOTORCH_VERSION = "0.6.4"
+PINNED_BOTORCH_VERSION = "0.6.6"
 
 if os.environ.get("ALLOW_BOTORCH_LATEST"):
     # allows a more recent previously installed version of botorch to remain