BUG: fix broken make_rarefaction_plots.py --generate_per_sample_plots option

Thanks @ebolyen for helping with this! Fixes #1475.
Merge branch 'master' into issue-1475

Conflicts:
	ChangeLog.md
Add @ebolyen to credits ecause he des
Merge pull request #1789 from jairideout/issue-1475

fix broken make_rarefaction_plots.py --generate_per_sample_plots option