Add augur version checking

[jameshadfield]

The snakemake file should store the version of augur it was tested against and compare this to the currently installed augur / containerized augur version via augur --version (see nextstrain/augur#146). The code for doing this should be somewhere central (see #2).

If the augur version is out-of-date then an error should be thrown and the snakemake file exits.

If the augur version is newer, then I think a warning is displayed but the build can proceed. If we exit on this mismatch, then all the pathogen repos have to be updated every time augur updates.

[tsibley]

#3 exists to consider this issue more generally (not just augur). This issue is a proposal for one approach to handling augur specifically.