Use excerpt_more in Post Grid Module

[aurorasmiles]

https://github.com/presscustomizr/nimble-builder/blob/master/tmpl/modules/post_grid_module_tmpl.php#L207
Instead of using excerpt_more, which a theme could override, this function uses a hardcoded […].
This makes changing the format of excerpt_more harder - I'd like to have […] link to the full post page