Enable/Disable Postgis without a table

[grimzy]

Currently, to use migrations to enable or disable Postgis requires a table:

Schema::create('some_table', function (Blueprint $table) {
    $table->enablePostgis();
});

This pull request enables the use of Schema::enablePostgis() and Schema::disablePostgis() directly on the schema which makes more sense.

[coveralls]

Changes Unknown when pulling 88c3d10 on grimzy:extension-on-schema into ** on njbarrett:master**.

[njbarrett]

This looks good, thanks for your contribution!