elt_divide(Matrix, var) not compiling

[bob-carpenter]

Summary:

Missing signature for elt_divide(Matrix, var).

Originally reported as: stan-dev/stan#1883

Description:

Missing definition for / needs to be replaced with Stan math divison.

stan/lib/stan_math/stan/math/prim/mat/fun/elt_divide.hpp:51:16: error: invalid operands to
      binary expression ('const Eigen::Matrix<double, -1, 1>' and 'stan::math::var')
      return m / s;
             ~ ^ ~

Reproducible Steps:

See: See: stan-dev/stan#1883

Current Version:

v2.9.0