refactor(client): No longer call fxa-js-client->signIn after signUp. signUp does everything it needs to do.

fixes #1840
Use the newest fxa-js-client which contains the `unwrapBKey` functionality needed in signUp.
Merge pull request #1963 from mozilla/issue-1840-no-signin-after-signup

refactor(client): No longer call fxa-js-client->signIn after signUp. signUp does everything it needs to do.