travis: Use GCC 4.9 for building
travis: Use GCC 4.9 for building

We need a compiler that supports C++14.

Signed-off-by: Pekka Enberg <add4fcd06328a394f0ad91feda7ee057316dc5ed@iki.fi>

jvirtanen/helix,penberg/helix,jvirtanen/helix,penberg/helix