Changelog for mwc-probability-2.0.3

# Changelog - 2.0.3 (2018-05-09) * Add inverse Gaussian (Wald) distribution - 2.0.2 (2018-01-30) * Add negative binomial distribution - 2.0.1 (2018-01-30) * Add Normal-Gamma and Pareto distributions - 2.0.0 (2018-01-29) * Add Laplace and Zipf-Mandelbrot distribution * Rename `isoGauss` to `isoNormal` and `standard` to `standardNormal` to uniform naming scheme * Divide Haddock in sections - 1.3.0 (2016-12-04) * Generalize a couple of samplers to use Traversable rather than lists.