Passing an array to dropForeign does not mean dropping multiple indices,
but rather dropping a key on multiple tables.
Passing a string means that this string will be interpreted as index
name, not as name of the indexed column. Passing an array with one
string is therefore correct, in order to benefit from automatic index
name generation.
The model classes encapsulate knowledge about the database that
may change. That knowledge may be table names, or the value of
constants (such as `Group::MEMBER_ID` in this case).
Models may even disappear after a while due to refactorings.
In all these cases, this migration would break. Thus, we use
Laravel's query builder features which makes building queries
almost as easy as using the model - but correct in all cases. :-)
