v1.5.19 added support for gap statistic on ordinations

phyloseq 1.5.19 - add gap statistic support for ordination results
===========
- gapstat_ord() - wrapper function for cluster::clusGap for ordination
methods
- plot_clusgap() - wrapper function for plotting gap statistic results
from clusGap()
- Full examples on the soilrep dataset for gap statistic.
- Fix bug in eigenvalue proportions for axes other
  than the first two in ordination plots.