Filter out pyenv from Travis path to fix python3.6 Travis errors

Summary:
This is a cherry pick of Core commit aa2e0f09ec by Corey Fields.
(ref. https://github.com/bitcoin/bitcoin/pull/11271)

Test Plan:
Check that Travis is no longer affected by error described here
https://github.com/travis-ci/travis-ci/issues/8391
and observed in recent ABC builds.

Reviewers: #bitcoin_abc, deadalnix, CCulianu, tomtomtom7

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D542