Use correct Python versions in CI environments
Use correct Python versions in CI environments

Currently, Travis CI runs one build for three major Python versions
except each environment installs and uses the same Python 3.6 version
from a fixed conda environment file. This change follows the
recommendations from the conda documentation [1] to build each conda
environment with the Travis Python version variable and the `conda
create` command instead of the `conda env create` command. The cost of
duplicating our dependencies here is small compared to the benefit of
testing Augur with the appropriate Python versions.

Fixes #626

[1] https://docs.conda.io/projects/conda/en/latest/user-guide/tasks/use-conda-with-travis-ci.html

nextstrain/augur,nextstrain/augur,blab/nextstrain-augur,nextstrain/augur