Fix Travis builds

setup.py test is deprecated and broken.  AFAIU it still uses
easy_install, which doesn't pay attention to python_requires of
dependencies, so when a transitive dependency drops Python 2.7 support
with a python_requires, pip can handle that, but easy_install can't.

I did not spend too much time investigating why exactly setup.py test
breaks: life's too short.  It's deprecated and that's reason enough to
stop using it.

I'm going to use pytest instead (because nose is also deprecated, lol).
I'm not going to switch to pytest wholesale because that would break
Windows builds (see #92).

.travis.yml

@@ -14,9 +14,11 @@ env:
     - FORCE_TEST_VCS=hg
     - FORCE_TEST_VCS=svn
 install:
-    - pip install coverage coveralls mock flake8 toml
+    - pip install -U pip
+    - pip install coverage coveralls pytest flake8
+    - pip install -e .[test]
 script:
-    - SKIP_NO_TESTS=1 coverage run setup.py test -q
+    - SKIP_NO_TESTS=1 coverage run -m pytest
     - coverage report -m
     - python check_manifest.py
     - flake8 *.py