Release procedure for shub

The Travis build is configured to release shub to PyPI whenever a new tag is committed.

The steps to do a release are:

Install bumpversion
Make sure you're at the tip of master, and then run:
```
bumpversion VERSION_PART
```
In place of VERSION_PART, use one of patch, minor or major, meaning the part of the version number to be updated.

This will create a new commit and tag updating the version number.
Push the changes and the new tag to trigger the release:
```
git push origin master --tags
```
Once the build finishes, run pip install shub in a temporary virtualenv and make sure it's installing the latest version.
Update the release information at:
```
https://github.com/scrapinghub/shub/releases
```
Travis and AppVeyor will automatically create a release draft and attach the binaries they built to it. Sometimes their timing leads to them creating multiple release drafts, in which case we need to combine them such that we have one release that has all three (Linux, OSX, Windows) binaries.

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