Fix compilation of json_unit with GCC 5

Addressing:
```
+ make json_unit
g++ -std=c++11  -Wall -Wextra -pedantic -Weffc++ -Wcast-align -Wcast-qual -Wctor-dtor-privacy -Wdisabled-optimization -Wformat=2 -Winit-self -Wmissing-declarations -Wmissing-include-dirs -Wold-style-cast -Woverloaded-virtual -Wredundant-decls -Wshadow -Wsign-conversion -Wsign-promo -Wstrict-overflow=5 -Wswitch -Wundef -Wno-unused -Wnon-virtual-dtor -Wreorder -Wdeprecated -Wfloat-equal  -I src -I test -Dprivate=public test/unit.cpp  -o json_unit
In file included from test/catch.hpp:65:0,
                 from test/unit.cpp:11:
/usr/include/c++/5.0.0/sstream:300:7: error: 'struct std::__cxx11::basic_stringbuf<_CharT, _Traits, _Alloc>::__xfer_bufptrs' redeclared with different access
       struct __xfer_bufptrs
```

Currently only support iterator and const_iterator. reverse_iterator
and const_reverse_iterator to be implemented soon.

Parse streams incrementally.

Feature/small float serialization