Removed scipy dependency from image_data_generator.

[eli-osherovich]

Summary

Related Issues

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)

This first step of removing scipy dependency (#322). It removes scipy from image_data_generator. Next steps will remove it from additional places.