binder fails

Currently, the binder build fails when executing the first cell. 
```c++
// setup matplotlib-cpp integration
#include "setup.h"
```
results in
```
In file included from input_line_8:1:
./setup.h:22:10: fatal error: 'xeus/xjson.hpp' file not found
#include "xeus/xjson.hpp"
         ^~~~~~~~~~~~~~~~

Interpreter Error: 
```