v1.5.23 update dependency from igraph0 to igraph package.

phyloseq 1.5.23 - Update dependency from igraph0 to igraph
===========
- Requested by CRAN
- igraph0 is being deprecated, removed from CRAN
- igraph is actively updated/maintained. igraph0 isn't / soon won't be.
- The load bug that I could find from re-running the tutorial in the
new dependency
  had to do with accessing the vertex names. The appropriate function
is:
  `get.vertex.attribute`
  which was not used previous, but is now imported and used in this
version of phyloseq.
- This also closes [Issue
247](https://github.com/joey711/phyloseq/issues/247)