bumped max pandas version to match qiime2-2019.10

Merge pull request picrust#96 from picrust/2.2.0

2.2.0

Merge pull request picrust#82 from picrust/ex_command_update

Minor changes to example commands in scripts

Merge pull request picrust#79 from picrust/reproduce

v2.1.3-b

Merge pull request picrust#74 from picrust/qiime2-2019.1_compatiblity

Qiime2 2019.1 compatiblity

Merge pull request picrust#70 from picrust/add_new_ref

Add alternative reference files

Updated README to reflect that placement tools are no longer packaged

Merge pull request picrust#47 from picrust/test_fix

Bioconda test fix and add GAPPA and EPA-NG as requirements

Merge pull request picrust#40 from picrust/custom_tree_pipeline

removed underscores from genome names