supply a single source for the version number

[duhaime]

Summary

Right now the keras_preprocessing version number is hardcoded in setup.py and the package's root level __init__.py file. This makes it cumbersome to change version numbers for development and testing purposes.

This PR uses one of the recommended "single source version number" patterns to supply just a single version number.

Related Issues

N/A

PR Overview

• [n] This PR requires new unit tests [y/n] (make sure tests are included)
• [n] This PR requires to update the documentation [y/n] (make sure the docs are up-to-date)
• [y] This PR is backwards compatible [y/n]
• [n] This PR changes the current API [y/n] (all API changes need to be approved by fchollet)