Conditionally load steller.js

[jamesros161]

ISSUE: Resolves #517

PROJECT: PPB Issues

Conditionally load steller.js

Test Files

post-and-page-builder-premium-1.1.4-issue-517.zip

NOTES TO TESTER

Please leave comments regarding issues found in testing directly on the issue linked above, not the Pull Request. This helps ease the process of reviewing the testing of multiple PRs in a given project from the project view.

Please remember to move this item from 'Needs Review' to either 'In Progress' or 'Awaiting Release' depending on the results of your testing

Testing Instructions

1. Install the test zip files linked above.
2. View a page that does NOT utilize the parallax background feature.
3. Use the Chrome Dev Tools to view the list of JS files loaded ( this can be done under the 'Network' tab of the dev tools )
4. Ensure that the file jquery.stellar.js is not loaded.
5. Edit the page to include a parallax background somewhere on the page.
6. Reload the page and ensure that the jquery.stellar.js file IS loaded and that the parallax effect works as expected.