update webtest version in test-requirements

run run_tests.sh or tox -epy27 may get many failures.
The reason is that the webtest is not the latest version. webtest check the environment:

In latest webtest:
assert type(environ[key]) in METADATA_TYPE, (
             "Environmental variable %s is not a string: %r (value: %r)"
             % (key, type(environ[key]), environ[key]))

In old webtest:

assert type(environ[key]) is str, (
             "Environmental variable %s is not a string: %r (value: %r)"
            % (key, type(environ[key]), environ[key]))

The latest webtest is 2.0.7, designate it in test-requirements.txt.