Makefile: fix `make docs`

Add bin/pip as prerequisite for sphinx-build and as target itself,
provided by installing a virtualenv.
docs/helpers: fix references to Client and RequestFactory
Makefile: test: only install requirements once or when updated
test_sole_test: fix fnmatch_lines for pytest-2.6.0-dev

With pytest 2.6 the output format for `-v` changes:

    from: tpkg/test_the_test.py:8: TestFoo.test_foo PASSED
    to:   tpkg/test_the_test.py@8::TestFoo::test_foo PASSED
doc: typo: s/a tests/a test/
Update Django versions for tox/travis
Merge pull request #114 from blueyed/update-tox-versions

Update Django versions for tox/travis
Merge pull request #113 from blueyed/doc-typo

doc: typo: s/a tests/a test/
Merge pull request #111 from blueyed/makefile-requirements-once

Makefile: test: only install requirements once or when updated
Merge pull request #104 from blueyed/fix-test_sole_test-for-pytest26

test_sole_test: fix fnmatch_lines for pytest-2.6.0-dev
Merge pull request #96 from blueyed/doc-fix-ref-rf-client

docs/helpers: fix references to Client and RequestFactory
Merge pull request #95 from blueyed/fix-makefile-bin-pip

Makefile: fix `make docs`
Make sure AuthenticationMiddleware is defined in settings
during tests.
Run tests against Django 1.7rc1
Avoid running django-configurations test on Django master
Added keepdb and serialize args to create_test_db_with_reuse
Remove the XFrameOptionsMiddleware from settings
Ignore the local directory when running tests. Refs #110
Provide a default for DJANGO_SETTINGS_MODULE in Makefile. Refs #110
Add failing tests: test_ds_after_user_conftest