change reshape2::melt to tidyr::gather

[MarcinKosinski]

reshape2 namespace has many conflicts with the dplyr package so I advise not to encourage users to use reshape2 anymore, so don't make them install this package because you just need melt in 3 places of your package. use tidyr::gather instead which some from the same universe as dplyr that you are using so far

https://stackoverflow.com/questions/26536251/comparing-gather-tidyr-to-melt-reshape2

@durszlaczek