Remove pip <8 version pinning

The pip issue pypa/pip#3384 is already been fixed after pip 8.0.1
released.

TrivialFix

Change-Id: Ib5bdff63a46ade8da31ddf4516affad6cf1a8029

scripts/env-setup.sh

@@ -54,7 +54,7 @@ if ! which pip; then
     sudo -H -E python /tmp/get-pip.py
 fi
 
-sudo -H -E pip install "pip>6.0,<8.0"
+sudo -H -E pip install "pip>6.0"
 sudo -H -E pip install -r "$(dirname $0)/../requirements.txt"
 
 u=$(whoami)