[WIP] Exit if pip version is detected to be too old.

.travis.yml

@@ -11,6 +11,14 @@ matrix:
     - sudo: 9000
       dist: trusty
       script:
+         - which python
+         - which pip
+         - which virtualenv
+         - virtualenv --version
+         - pip install --upgrade pip virtualenv
+         - which pip
+         - which virtualenv
+         - virtualenv --version
          - ./mach build -d --verbose
          - ./mach test-compiletest
          - ./mach test-unit

python/mach_bootstrap.py

@@ -8,6 +8,8 @@
 import platform
 import sys
 from distutils.spawn import find_executable
+from distutils.version import LooseVersion
+import subprocess
 from subprocess import PIPE, Popen
 
 SEARCH_PATHS = [
@@ -117,6 +119,11 @@ def _activate_virtualenv(topdir):
         if not virtualenv:
             sys.exit("Python virtualenv is not installed. Please install it prior to running mach.")
 
+        virtualenv_version = subprocess.check_output([virtualenv, "--version"])
+        print("virtual env version = {}".format(virtualenv_version))
+        if LooseVersion(virtualenv_version) < LooseVersion("14.0.0"):
+            sys.exit("virtualenv is too old. Please upgrade to at least version 14.0.0.")
+
         process = Popen([virtualenv, "-p", python, virtualenv_path], stdout=PIPE, stderr=PIPE)
         process.wait()
         if process.returncode: