ENH: implement non-unique indexing in series (GH4246)

DOC: release notes
Merge pull request #4247 from jreback/series_dups

API: implement non-unique indexing in series (GH4246)
BLD: use mpl 1.1.1 in python 2.7 production travis build
Merge pull request #4243 from kjordahl/master

DOC: Fix typos in CONTRIBUTING.md
Merge pull request #4248 from cpcloud/mpl-1.1.1-build

BLD: use mpl 1.1.1 in python 2.7 production travis build