Add python 3 back in to travis CI

[adamrp]

Removed because biom was not python 3 compatible (see biocore/biom#535), which is breaking the qiita build when testing with python 3.

However, when biom becaomse python 3 compatible, we should add it back in here, too.

[adswafford]

Haha, so it looks like the py3 upgrade has been in the work for 3 years now... Flagging for input so we can prioritize appropiately.