Fix mlpack dependency on C++11 library versions

Hello,

I recently ran into an issue installing mlpack on OS X 10.8 using the --c++11 option.  It appears that the mlpack dependency on armadillo and boost which requires the C++11 versions of those libraries needs to use the string 'c++11' rather than 'cxx11' in the dependency array.

Although the symbol for the option and build property are cxx11, the dependency string must be "c++11" (for example, see graph-tool and insighttoolkit).  Otherwise installation fails with:

    Error: mlpack dependency armadillo not installed with:
      --cxx11

Even when armadillo was compiled with --c++11.

Cheers,
Kevin