Updates travis/tox to test at minimum supported versions

[paulray]

This updates our travis/tox testing to run tests on the minimum supported versions of numpy and astropy to ensure that new features don't break support for the older versions.
In order to make this work, I had to bump up the minimum version for numpy to 1.15.*

Fixes #632 and #584

[paulray]

Note that our use of travis and tox is a bit of an unholy mess. It would be nice to use tox for all our test runnings, following the nicely organized example of astropy. This should be a separate issue/feature request.

[rossjjennings]

It would be nice to have a Python 2 version of this test as well. I suspect it will show that we need to bump the minimum version of astropy to 2.0.15.

[paulray]

True I'll add a Python2 version

[paulray]

OK, I have tried but I can't make the python2 test work in travis-ci. So, it is commented out of .travis.yml. If you want to run it on your local machine you can do tox-2.7 -e oldestdeps2. That runs fine on my machine. But, it is not that interesting since the oldest deps under Python 2.7 are very close to the current versions, since astropy got stuck at 2.x under Python 2.7.

Therefore I propose that this be merged now. This bumps up the minimum versions for astropy and numpy and puts in place a test that will ensure that (under Python 3.x at least) we don't introduce changes that break under the oldest supported versions.

[paulray]

@scottransom or @luojing1211 if you are happy with this, please merge.

[luojing1211]

So which version is the last py2.7 supported version? The original plan was PINT 0.7, the next one, should be the last one.

[paulray]

That still sounds like a good plan to me. When we release version 0.7, the one that matches the paper, we can then move forward and say that versions 0.8 and forward to NOT try to support Python 2.

[luojing1211]

Should we merge this PR later, since it does not test PY2.7

[paulray]

No, it DOES test Python 2.7, as it always did. What is missing is @rossjjennings request to ALSO test Python 2.7 with the minimum supported versions of numpy and astropy. But, we've never had that anyway. This PR at leasts gives us that capability for Python 3.

[luojing1211]

Ok, I saw the tests. I think we can merge it.