Update requirements.txt

Author: mihaimaruseac

research/multi_representation_adversary/requirements.txt

@@ -3,6 +3,6 @@ git+git://github.com/cleverhans-lab/cleverhans@v3.1.0
 gin-config
 neural-structured-learning>=1.3.0
 numpy~=1.19.2
-tensorflow~=2.4.1
+tensorflow~=2.6.0
 tensorflow-addons
 tensorflow-datasets>=4.0.0