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!
Merge pull request #606 from sass/test/travis-ci

[WIP] Fixes and improves travis ci build
Removes redundant builds in travis-ci config

Further speeds up day by day travis-ci builds
This will still catch all coding errors!
Support function and mixin calls with both variable and keyword args
Reevaluate var and keyword arg flags after eval a \$variable argument

```
foo($bar...);
```

Will flag `$bar` as a variable arg when in fact it may be a keyword
arg. We can only be sure once `$bar` has been evaluated. If `$bar`
is evaluated to a map then it's a keyword argument and the
Argument object needs to be updated accordingly.
Merge pull request #611 from xzyfer/feat/map-varargs

Add support for keyword arguments