Filter out pyenv from Travis path to fix python3.6 Travis errors
This is a cherry pick of Core commit aa2e0f09ec by Corey Fields.
Check that Travis is no longer affected by error described here
and observed in recent ABC builds.
Reviewers: #bitcoin_abc, deadalnix, CCulianu, tomtomtom7
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D542