Disable the pip version check message

This message is being shown to people who should not be deciding whether
to upgrade pip, so it is just confusing and alarming noise.

Before:

---> pavelib.prereqs.install_prereqs
---> pavelib.prereqs.install_ruby_prereqs
Ruby prereqs unchanged, skipping...
---> pavelib.prereqs.install_node_prereqs
Node prereqs unchanged, skipping...
---> pavelib.prereqs.install_python_prereqs
pip install -q --exists-action w -r requirements/edx/pre.txt
You are using pip version 6.0.8, however version 7.1.2 is available.
You should consider upgrading via the 'pip install --upgrade pip' command.
pip install -q --exists-action w -r requirements/edx/github.txt
You are using pip version 6.0.8, however version 7.1.2 is available.
You should consider upgrading via the 'pip install --upgrade pip' command.
Could not find a tag or branch '96e1922348bfe6d99201b9512a9ed946c87b7e0b', assuming commit.
Could not find a tag or branch 'b0686a76f1ce3532088c4aee6e76b9abe61cc808', assuming commit.
Could not find a tag or branch '88ec8a011e481918fdc9d2682d4017c835acd8be', assuming commit.

After:

---> pavelib.prereqs.install_prereqs
---> pavelib.prereqs.install_ruby_prereqs
Ruby prereqs unchanged, skipping...
---> pavelib.prereqs.install_node_prereqs
Node prereqs unchanged, skipping...
---> pavelib.prereqs.install_python_prereqs
pip install -q --disable-pip-version-check --exists-action w -r requirements/edx/pre.txt
pip install -q --disable-pip-version-check --exists-action w -r requirements/edx/github.txt
Could not find a tag or branch '96e1922348bfe6d99201b9512a9ed946c87b7e0b', assuming commit.
Could not find a tag or branch 'b0686a76f1ce3532088c4aee6e76b9abe61cc808', assuming commit.
Could not find a tag or branch '88ec8a011e481918fdc9d2682d4017c835acd8be', assuming commit.

pavelib/prereqs.py

@@ -138,7 +138,7 @@ def python_prereqs_installation():
     Installs Python prerequisites
     """
     for req_file in PYTHON_REQ_FILES:
-        sh("pip install -q --exists-action w -r {req_file}".format(req_file=req_file))
+        sh("pip install -q --disable-pip-version-check --exists-action w -r {req_file}".format(req_file=req_file))
 
 
 @task