Travis: use -std=gnu99 with NSS, and clang with Chutney

NSS:
* test NSS-specific code with -std=gnu99
* use a recent gcc version from the latest Ubuntu image

Chutney:
* use clang, so we keep one clang Linux job
* keep clang on a fast job, so the overall build finishes quickly

Closes ticket 32500 for 0.3.5.