Deprecate dynamic::dynamic(std::initializer_list<dynamic>)

Deprecate dynamic::dynamic(std::initializer_list<dynamic>)

Summary:After DR95 the single braces dispatch to the copy constructor
(http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1467),
so it is not possible anymore to initialize a singleton dynamic array
using the braces syntax. The initializer list constructor already had
a special case for empty lists, which unconditionally called the
default constructor if defined.

This diff deprecates the braces syntax and defines the following alternative:
```
dynamic empty = dynamic::array;
dynamic a = dynamic::array(1, 2, "foo");
```

Reviewed By: luciang, yfeldblum

Differential Revision: D3013423

fb-gh-sync-id: a0cf09dfd96e9806044f7c3bf3592c637a9bc932
shipit-source-id: a0cf09dfd96e9806044f7c3bf3592c637a9bc932