Document libffi-dev as a compile-time requirement

Without libffi-dev (Ubuntu 14.04, vagrant image, 64bit), I get the following error

```
[translation:ERROR] CompilationError: CompilationError(err="""
[translation:ERROR]     /tmp/usession-default-1/platcheck_39.c:35:17: fatal error: ffi.h: No such file or directory
[translation:ERROR]      #include <ffi.h>
[translation:ERROR]                      ^
[translation:ERROR]     compilation terminated.
[translation:ERROR]     """)
```

Installing libffi-dev removes this error and allows compilation to succeed.