Upgrade pip in the venv when we build it.

Change-Id: I924975170c9e3a1ef23b5195cb3a6eed675ec043

tools/install_venv.py

@@ -150,6 +150,9 @@ def pip_install(*args):
 def install_dependencies(venv=VENV):
     print 'Installing dependencies with pip (this can take a while)...'
 
+    # First things first, make sure our venv has the latest pip.
+    pip_install('pip')
+
     # Install greenlet by hand - just listing it in the requires file does not
     # get it in stalled in the right order
     pip_install('greenlet')