Improve dependencies compatibility definition, and testing. Fix #75

[thibaudcolas]

Fixes #75.

The only user-facing changes are:

• The new upper version bound for lxml, which wasn't defined before.
• The new upper version bound for html5lib, which used to be lower (now that htm5lib has been released with good version numbers).

This PR also updates the tox environments, as described in #75.

[thibaudcolas]

This change had a similar issue to #88. It seems different lxml (or libxml2?) versions produce a different output. For a specific sequence of unescaped HTML characters (<p>Invalid < " > &</p>, or potentially something as simple as <p>Invalid < 3</p>).

Trying to isolate this, it the difference seems to be between v3.7.1 and below, vs v3.7.2 and up:

• 3.7.2 (and up to latest) fail: https://travis-ci.org/springload/draftjs_exporter/jobs/378159145
• 3.7.1 (and down to 3.6.0) success: https://travis-ci.org/springload/draftjs_exporter/jobs/378159146

I'll try to repro this further and report it on lxml or libxml.