[SECP256K1] travis: Explicitly set --with-valgrind
Summary:
Also remove CPPFLAGS=-DVALGRIND because that's redundant with when configured with --enable-valgrind.
Backport of secp256k1 PR821.
Test Plan:
Run the Travis build.
https://travis-ci.org/github/Fabcien/secp256k1/builds/731225830
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Subscribers: deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D7633