travis-ci: drop nightly, no alternative for 7.4 anymore
travis-ci: drop nightly, no alternative for 7.4 anymore

Bit-Wasp/secp256k1-php,Bit-Wasp/secp256k1-php,Bit-Wasp/secp256k1-php,Bit-Wasp/secp256k1-php,Bit-Wasp/secp256k1-php