Update Python version for ghost-tree compatibility with QIIME 2 2019.1 release

[JTFouquier]

ghost-tree updates for QIIME 2019.1 release

• Fix flake8 code errors in Python 3.6 build (flake8 checks for Python Pep8 style)
• Remove Python 3.4 support, fix 3.6 support and possibly keep 3.5
• Talk to Elmar about bumping version for new Conda release that supports only 3.6 (maybe 3.5)
• Review the ghost tree Conda ghost-tree package, says it supports >= 3.4 but Travis is unhappy with 3.4 and 3.6 for some reason despite approval of my commit in September 2018.

[JTFouquier]

@emastej there actually were a few issues that were randomly caught when I made that example code push to close the issue on the issue tracker. These changes had to be done anyways due to a new QIIME 2 2019.1 release.🙂

[JTFouquier]

I believe init.py can be removed. Originally code was written in Python 2, but since the Python 3 version, I do not think this is required. Look into later.