[SECP256K1] Travis : run a 64 bits ninja for building 32 bits targets

Summary:
There is no reason for using the 32 bit executable since the host is 64
bits.

Test Plan:
Run the Travis build (see
https://travis-ci.org/Fabcien/secp256k1/builds/642818632).

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5087