This copies the official Docker Python images approach of stripping
the Python test suite after building from source.
For Python 3.6.2 this reduces the output from 184MB to 121MB and
removes the following directories:
```
.heroku/python/lib/python3.6/tkinter/test
.heroku/python/lib/python3.6/ctypes/test
.heroku/python/lib/python3.6/unittest/test
.heroku/python/lib/python3.6/distutils/tests
.heroku/python/lib/python3.6/sqlite3/test
.heroku/python/lib/python3.6/lib2to3/tests
.heroku/python/lib/python3.6/test
```
For Python 2.7.13 this reduces the output from 127MB to 91MB and
removes the following directories:
```
.heroku/python/lib/python2.7/ctypes/test
.heroku/python/lib/python2.7/unittest/test
.heroku/python/lib/python2.7/distutils/tests
.heroku/python/lib/python2.7/bsddb/test
.heroku/python/lib/python2.7/lib-tk/test
.heroku/python/lib/python2.7/sqlite3/test
.heroku/python/lib/python2.7/lib2to3/tests
.heroku/python/lib/python2.7/json/tests
.heroku/python/lib/python2.7/email/test
.heroku/python/lib/python2.7/test
```
Fixes#424.
* Updates bob-builder to a version that clears out the previous
build directory, which prevents the stale files seen in #379.
* Adds a `buildenv-heroku-16` Make command to simplify the
building and use of the Heroku-16 binary build environment.
* Sets `S3_BUCKET` and `S3_PREFIX` in the Docker image, to save
having to do so manually each time.
* Removes the duplication of the bob-builder dependency between
`Dockerfile` and `requirements.txt`.
* Disables the pip version check during Docker build, to avoid the
noisy stderr warning about Ubuntu 16.04 pip being older (v8.1.1).
* Adds a `.dockerignore` to speed up the Docker build, by reducing
the build context transferred to the daemon from 60MB to 2MB.
* Applies some Dockerfile best practices like disabling/removing
cached files (since layer invalidation makes caching pointless).
Johannes Hoppe
kennethreitz