install newly required header in autotools build system

sass_interface.h #includes sass2scss.h so we need to have the
latter header wherever we have the former one.
Merge pull request #609 from idupree/autotools-header-install-fix

install newly required header in autotools build system
Fixes and improves travis ci build

Changes -std=c++11 to -std=c++0x!
Test travis-ci