The --format flag is only in pip 9 and later

So lets make that an implicit dependency.

git-svn-id: https://llvm.org/svn/llvm-project/zorg/trunk@324138 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/dep/dep.py b/dep/dep.py
index 3697177..d19e021 100644
--- a/dep/dep.py
+++ b/dep/dep.py
@@ -539,7 +539,16 @@
 
     def verify(self):
         """Verify the packages in pip match this dependency."""
+
         try:
+            pip_version = subprocess.check_output(["/usr/bin/env", "python", "-m", "pip", "--version"])
+            pip_tokens = pip_version.split()
+            assert pip_tokens[0] == "pip"
+            pip_version = Version(pip_tokens[1])
+
+            if pip_version < Version("9.0.0"):
+                raise MissingDependencyError("Version of pip too old.")
+
             pip_package_config = json.loads(subprocess.check_output(["/usr/bin/env",
                                                                      "python", "-m", "pip", "list", "--format=json"]))
         except (subprocess.CalledProcessError, OSError):