Remove support for deprecated v1 API

Fixes #7.

Removed from client:
 - start_authenticate_v1
 - finish_authenticate_v1
 - CredentialsV1

Removed lxml as dep, v1 tests, and references in readme.

Ran tests (thank you for the awesome test suite!):

Name         Stmts   Miss  Cover   Missing
------------------------------------------
box              3      0   100%
box.client     216      0   100%
------------------------------------------
TOTAL          219      0   100%
----------------------------------------------------------------------
Ran 51 tests in 0.039s