Fix neg_binomial_rng
fully specifying call to boost::math::pdf
updated neg_binomial_rng test to use the neg_binomial_log to generate the expected values
updating travis script
Merge pull request #601 from stan-dev/feature/travis-ci

updating travis script
Merge pull request #594 from stan-dev/feature/fix-issue591-nb_rng

Fixes #591. neg_binomial_rng
Merge branch 'develop' of https://github.com/stan-dev/stan into develop