travis CI CHANGE use new Trusty environment
travis CI CHANGE use new Trusty environment

- use Trusty environment with Ubuntu 14.04 (faster environment)
- add gcc compilation besides clang
- use empty build directory to build libyang (so do it as described in README)

PavolVican/libyang,PavolVican/libyang,PavolVican/libyang,lukasmacko/libyang,PavolVican/libyang,PavolVican/libyang,lukasmacko/libyang