Pin pip to <19 until a more permanent solution can be devised. Ref #1644

.

pip-wrapper.py

@@ -3,14 +3,23 @@
 
 - Avoid the default 'python -m pip' invocation, which causes the current
   working directory to be added to the path, which causes problems.
+- Ensure pip meets a requisite version.
 """
 
 
 import sys
 import subprocess
 
 
+def ensure_pip_version(pip_bin, ver):
+    cmd = [pip_bin, 'install', 'pip ' + ver]
+    subprocess.check_call(cmd)
+
+
 def main():
+    pip_bin = sys.argv[1]
+    # workaround for #1644
+    ensure_pip_version(pip_bin, '<19')
     cmd = sys.argv[1:]
     subprocess.check_call(cmd)