Upgrade virtualenv for bundled installers

The upgrade is needed due to the version of wheels pulled
in. When pulling in PyYAML as a dependency, PyYAML tries to
import a function from wheels that throws an IOError on
some patched Ubuntu versions. The issue is similar to:
pypa/pip#1074

Wheels has a fix or that bug and virtualenv
pulls that version of wheels in so it should no longer be an
issue.

scripts/make-bundle

@@ -24,7 +24,7 @@ from contextlib import contextmanager
 # the installation process.  It is *not* the deps required
 # by awscli/botocore.
 PACKAGE_VERSION = {
-    'virtualenv': '13.0.3',
+    'virtualenv': '15.1.0',
     # These packages are included because they are required for
     # python2.6, but our normal dependency downloading via pip
     # only works if you use python2.6.  To fix this issue, we're