I have access to a machine with CUDA 7.5, but I think it is missing the cuSolverRF library.
The current documentation marked as version DU-06709-001_v8.0 says "In addition cuSolver provides a new refactorization library useful for solving sequences of matrices with a shared sparsity pattern" presumably the cuSolverRF library, suggesting that it was introduced in 8.0, but I can't find any release notes confirming this.
the cuSolver library was introduced in CUDA 7.
the cuSolverRF features of cuSolver were introduced during the CUDA 7 release cycle. i.e. they have been part of cuSolver since day 1.
Here is the CUDA 7 cuSolver reference manual supporting this assertion.