Prepare new release.

Update version number.

Update version number.

Remove unused pandas import (keras-team#156)

### Summary

At the moment there is an unused pandas import. This is also the only import of pandas in the source code which makes the import of the package break if the user hasn't pandas installed. This PR removes the import.

### Related Issues

keras-team#154 

### 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)

Update version number to 1.0.6.

Prepare 1.0.5 release.

Prepare 1.0.4 release.

Merge pull request keras-team#37 from juharris/pad_str

pad_sequences: Add support for string value.

Merge pull request keras-team#28 from kaczmarj/patch-1

docs: minor spelling fix in `.apply_transform`

Prepare 1.0.1 release