ENH/TST: Add check that example tables are valid

Adds a line to .travis.yml that runs `biom summarize-table` on all the `*.biom`
files inside examples. With the aim of preventing those tables to become
outdated.

I had two remove two tables that couldn't be parsed, can someone re-generate
these and add them to the repo, culprits:

- min_dense_otu_table.biom
- rich_dense_otu_table.biom

Fixes #364