Allow for newer versions of gcc to compile MeTA.

Tested with GCC 4.8. This is mostly a "hack" in the sense that it falls
back on some things that are in theory less performant for the sake of
getting it to work. It's not a hack in the sense that these features are
detected with CMake and eventually we can eliminate them when GCC catches
up with the standard...
fix formula in absolute discount method
add ranker tests; still need to investigate absolute discount errors
Change remote for cpptoml to github remote.
Change porter2_stemmer remote to github.
Add travis-ci integration.
Update travis-ci configuration to build libc++abi for clang.
Copy over includes for libc++abi building.
Try to build libc++ first in travis-ci.
Fix typo for -D flag for libc++ in travis.yml.
Attempt to get libc++abi to build first for travis-ci.
Attempt to use default C++ abi for travis-ci.
Add travis-ci badge to readme.
Merge branch 'develop'