Better detect unpacked eggs

Do not assume a directory named in `.egg` is an egg, unless it has an
actual egg metadata directory.

Closes #462

But our tests do, one that provides `--no-wheel`.

Directory used by setuptools to auto-install dependencies

travis: fix deployment

update Travis builds

tests: disable pytest-flake8 on Python 2.6

Fix `test` command handling of `extras_require`

Python 2.6 support has been official dropped from
flake8 for some time, and broken since 3.4.0.

Update to the future defaults: `dist: trusty`.

Also install platform specific requirements in `extras_require`.

do not strip empty sections in `extras_require`

Revert "fix `install_requires` handling of extras"

This reverts commit a3ec721e.

Remove dependency on virtualenv. Setuptools' tests don't require it except pytest-virtualenv, which declares it.