[5.6] Support JSON SELECT queries on SQLite

[staudenmeir]

Adds JSON SELECT queries to SQLite:

DB::table('users')->select('items->price')->where('items->price', 1)->orderBy('items->price');

In order for this to work, the JSON1 extension has to be enabled. That's the case in the latest versions of Homestead.

Users without the extension will get an error: General error: 1 no such function: json_extract
Of course, that's not really helpful. But to detect this beforehand, we would have to run an extra pragma compile_options query and look for ENABLE_JSON1.

There is apparently no way to support whereJsonContains().

There is no integration test because the JSON1 extension isn't available on Travis CI (I tried).