Improve package dependency management

[forman]

Dependencies should be expressed such that PACKAGE>=MAJOR.MINOR[.PATCH]<MAJOR.(MINOR+1) achieving API compatibility according to SemVer. PATCH should ideally be omitted if not required for some reason.

In only rare cases we should rely on a specific version such that PACKAGE=MAJOR.MINOR.PATCH. For example, this could be the consequence of fixing a dependency conflict. In any of such cases, the reason for doing so must be documented (e.g. comment line).

If our dependency configuration is too strict, cate will tend to downgrade already installed packages when installed into a user's existing Python environment.

Transitive dependencies should not be specified at all unless there is special need for this. A dependency is non-transitive if we import it explicitly in Python code or we need it for some special behaviour (e.g. dask used by xarray) or it provides a special, dynamically loaded plugin.

These are the places where dependencies are declared. All configurations should share the same dependency specifications:

• setupy.py
• .travis.yml
• environment.yml, currently used by Sphinx only
• Conda recipes in cate-conda
• Constructor installer configuration in cate-installer

This issue is directly related to #131.

[forman]

While fixing this, we also should try moving to Python 3.6 to be up to date.