Travis config not fully consistent with setup.py

[kandersolar]

Here are two proposed improvements to our .travis.yml.

---

First, Travis doesn't run pip install rdtools[test] during the job setup. It's not currently a problem, I guess because Travis happens to supply the current requirements (like pytest) out of the box, so the tests pass. But if we add a new requirement to [test] (for example, tables for #131), the requirements aren't met and the tests fail.

I think changing these lines to REQ_ENV='<...> .[test]' will have the desired effect:

rdtools/.travis.yml

Lines 14 to 16 in 8f00cd4

	env:
	- REQ_ENV='-r requirements.txt .'
	- REQ_ENV='--upgrade --upgrade-strategy=eager .'

---

Second, the minimum dependency versions specified in our setup.py aren't tested in the CI which makes it more likely for us to unintentionally break compatibility with the minimum supported versions. I propose either: 1) changing requirements.txt to match our minimum supported versions, or 2) creating a requirements-min.txt with the minimum versions specified in setup.py and adding a third Travis env entry to use it for tests. I'd prefer option 2 because requirements.txt is user-facing and I don't really think we should be encouraging people to use the oldest package versions possible.

[cdeline]

OK, I agree with this approach. I'll create a feature branch to test these and merge into development to check. I can merge back into modelchain pytests to see if it fixes things.

[cdeline]

Actually, on second thought I'm out of my depth here. I'll leave it to you @kanderso-nrel, but I agree with this approach.

[mdeceglie]

Closed with #214