Base Django images out of Alpine 3.3

[rvernica]

I made the necessary changes to base the Django images out of Alpine 3.3.

Here are a few notes:

1. The required database libraries for SQLite, MySQL and PostgreSQL are installed first.
2. pip install requires some -dev packages. I install the -dev packages, run pip and then remove the -dev packages. (I use the apk's virtual feature.) All this is done in a single Docker command. This keeps the images small. This is also done for onbuild images when the requirements.txt are installed.
3. For the Python 3 image, Alpine 3.3 comes with Python 3.5, so I changed the directory name from 3.4 to 3.5 and updated the Travis configuration file.
4. I tested the four images (2.7, 2.7-onbuild, 3.5, and 3.5-onbuild) by running a small project with Django Admin interface on SQLite, MySQL (MariaDB) and PostgreSQL back-ends.

[rvernica]

Build failed due to DNS issues with Alpine Linux. Their domain expired briefly and was re-registered shortly after. The build on my fork was successful.

[rvernica]

I built the images in my Docker Hub and their reported size is 18-37MB. Compare that to 158-280MB for the non-Alpine images.

[mwaaas]

would love to see this changes merged

[yosifkit]

We have deprecated the django image in favor of just the python image: docker-library/docs#630.