Drivers, Content: ignore sticky posts when fetching.

[paulgibbs]

Description

This prevents the underlying WP_Query in WP-PHP driver and WP-CLI itself returning sticky posts when retriving a post ID by post title, as seen with the "I am on the edit screen for" step definition.

This does not affect intentionally retrieving sticky posts in this way.

Related issue

Fixes #218

How has this been tested?

Tested locally.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.