Looking at the release notes, Simulink Verification and Validation was actually released in R2006b but with a limited functionality. So you can use the said toolbox with R2015a.
Unfortunately, it is not possible to use later releases of toolbox on a previous MATLAB release. So if you want to use all the current features in Simulink Verification and Validation, you would have to upgrade MATLAB to the latest release.
Following is a link for release notes for this particular toolbox: