Pin plumbum and six in Ubuntu build dockerfile.
Pin plumbum and six in Ubuntu build dockerfile.

Signed-off-by: Sergey Minaev <322af3f2df10918c6ef5280f56be0b711278b1ae@dsr-corporation.com>
Artemkaaas/indy-sdk,Artemkaaas/indy-sdk,Artemkaaas/indy-sdk,Artemkaaas/indy-sdk,Artemkaaas/indy-sdk,Artemkaaas/indy-sdk,Artemkaaas/indy-sdk,Artemkaaas/indy-sdk,Artemkaaas/indy-sdk,Artemkaaas/indy-sdk,Artemkaaas/indy-sdk,Artemkaaas/indy-sdk,Artemkaaas/indy-sdk