FEAT: add a clunky freeze/requirements-like feature to enpkg.
BUG: ensure we don't resolve dependencies in --requirements mode.

The major point of this feature is to bypass the dependency mechanism
when we have incompatible dependencies.
BUG: let's not modify module-level globals in tests...
BUG: use mock instead of modifying module-level globals.
BUG: fix old tests to work on 2.6 as well in randomized mode.
MAINT: use randomize when running tests on travis-ci.
TST: add tests for config.get_path.
TST: add tests for input_auth.
TST: add basic tests for config.write.
TST: add test for web_auth.
BUG: fix 2.6 support.
TST: add tests for get_auth.
TST: add tests for change_auth.
TST: add tests for authenticate.
BUG: fix exception handling in authenticate when webservice is used.
TST: add tests for subscription level.
BUG: fix mocked_get default path.
Merge pull request #129 from enthought/freeze_hack

FEAT: add a clunky freeze/requirements-like feature to enpkg.
REF: don't use class-level variables per review request.
Merge branch 'master' into test_config_v2