Revert "Cap Pip<8 due to pip bug"

This reverts commit 308484592dd8bb993dc24a6d6aacefc611cd7e8b.
This issue has been fixed after pip 8.0.1 released.

TrivialFix

Change-Id: I193ef41c2a9efcab3aea54f4949cf6fb1e9b1084
diff --git a/tools/install_venv_common.py b/tools/install_venv_common.py
index d1643f8..e279159 100644
--- a/tools/install_venv_common.py
+++ b/tools/install_venv_common.py
@@ -113,7 +113,7 @@
 
         # First things first, make sure our venv has the latest pip and
         # setuptools and pbr
-        self.pip_install('pip<8,>=1.4')
+        self.pip_install('pip>=1.4')
         self.pip_install('setuptools')
         self.pip_install('pbr')