Save as IDML, then open in cc 2014
Just open the 2015 file in 2014. File > Package (which produces the IDML file mentioned above along with collecting and moving all of the assets to a new folder) is old stand-by (and definitely works), but Adobe recently added the ability for InDesign to open newer version files. For example, you can simply open a 2015 file in CS6 now.
From the online help: New features and enhancements in InDesign CC 2014:
Opening a newer version document
You can now send documents to anyone using older versions of InDesign without extra steps like manually exporting to IDML. Compatibility is built right in, ensuring that your documents always open just as you would expect.
If you open a document created in a newer version of InDesign and click OK in the following message, InDesign uses a Creative Cloud service to convert the document to the InDesign version that you are running. InDesign then saves the converted document back to your computer.