Makefile: Add pip version check

We use features of pip-6.0.1 in our requirement files, therefor we
depend on pip>=6.0.1. This commit adds this dependency before
requirements are processed without "--upgrade". When the user has
pip>=6.0.1, nothing happens. When he has older pip it's upgraded.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

Makefile

@@ -117,6 +117,7 @@ clean:
 	find . -name '*.pyc' -delete
 
 requirements:
+	- pip install "pip>=6.0.1"
 	- pip install -r requirements.txt --upgrade
 
 requirements-selftests: requirements