ENH: update to remove cogent dependency, fixes #1552
first pass as porting to use skbio's majority_rule
ENH: updated scikit-bio dependency version
Merge branch 'issue-1552' into issue-1551
Merge pull request #1618 from gregcaporaso/issue-1552

remove cogent neighbor joining dependency in favor of skbio
Merge pull request #1619 from gregcaporaso/issue-1551

porting to use skbio's majority_rule