Roll virtualenv 12.0->15.0.2

bootstrap/README.md

 # Bootstrapping infra.git
 
 [TOC]
 
 The [infra.git](/) repo uses python [wheel files][wheel files],
 [virtualenv][virtualenv] and [pip][pip] to manage dependencies. The process for
 bootstrapping these is contained entirely within [bootstrap/ directory](.).
 
 ## TL;DR - Workflows
 
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 See [custom builds](#Custom-builds) below for more detail.
 
 ## bootstrap.py (a.k.a. "I just want a working infra repo!")
 
 Run `gclient runhooks`. Under the hood, this runs:
 
     ./bootstrap/bootstrap.py --deps_file bootstrap/deps.pyl ENV
 
 This creates a virtualenv called `{repo_root}/ENV` with all the deps
 contained in `bootstrap/deps.pyl`. You must be online, or must already
-have the wheels for your system cached in `{repo_root}/.wheelcache`.
+have the wheels for your system in cache.
 
 If you already have an `ENV` directory, [bootstrap.py](bootstrap.py) will check
 the manifest in `ENV` to see if it matches [deps.pyl](#deps_pyl) (i.e. the diff
 is zero). If it's not, then `ENV` directory will be re-created *from scratch*.
 
 [run.py](../run.py) will automatically use the environment `ENV`. It is
 an error to use `run.py` without first setting up `ENV`.
 
 ## [deps.pyl](deps.pyl)
 
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 an additional component to the version field).
 
 For example, given the package `jane` at version `2.1.3`, we would
 create a branch `2.1.3-cr`. On this branch we would commit any changes
 necessary to `2.1.3`, and would adjust the version number in the builds
 to be e.g. `2.1.3.0`.
 
 [wheel files]: https://www.python.org/dev/peps/pep-0427/
 [virtualenv]: https://github.com/pypa/virtualenv
 [pip]: https://github.com/pypa/pip