std::string_view is slated for C++17 and there is already support for a std::experimental::string_view.
http://en.cppreference.com/w/cpp/experimental/basic_string_view

In addition, many projects have their own implementation of string_view.

I noticed your StringRef class contains a method called c_str(). If you are following the STL paradigm, the existance of this method means that your StringRef class has an invariant that must always reference a null terminated string and therefore the formatting functions require the format string to be null terminated.

Is this the case?

If not, I would suggest renaming StringRef::c_str() to StringRef::data(). I'd also add whether or not null termination is required on the format string to the documentation.

If you are planning to support non-null terminated format strings, a constructor accepting a std::experimental::string_view (conditionally enabled with the __cpp_lib_experimental_string_view macro) with also help people using string_view use your library.