make: explicitly restrict QA builds to 2048 RSA
make: explicitly restrict QA builds to 2048 RSA

2048 bit was the prior default for all builds, so it wasn't explicitly
set. With the new build system, the default is instead 3072 and
4096. Consequently, the tests took much longer to run and would
timeout on Travis.

miracl/milagro-crypto-c,philipp-cliqz/milagro-crypto-c,philipp-cliqz/milagro-crypto-c,philipp-cliqz/milagro-crypto-c,milagro-crypto/milagro-crypto-c,miracl/milagro-crypto-c,milagro-crypto/milagro-crypto-c,miracl/milagro-crypto-c,miracl/milagro-crypto-c,philipp-cliqz/milagro-crypto-c,milagro-crypto/milagro-crypto-c,milagro-crypto/milagro-crypto-c,miracl/milagro-crypto-c