Update travis build-config.
Update travis build-config.

- Build for node 0.10, 0.11, 0.12 and 4.0.
- Build with both gcc (4.4, 4.6, 4.8) and clang.
- Building for node 4.0 requires gcc 4.8, so 4.4 and 4.6 are disabled.

ndob/duktape-node,ndob/duktape-node,ndob/duktape-node