Update core.py

There's a problem here:

![open_link_fail](https://f.cloud.github.com/assets/390964/323692/e71d469e-9ac4-11e2-828f-a6376a33f322.gif)

The url that tries to open is from the api subdomain:

 https://api.github.com/repos/alejandrogomez/shipit/issues/29

related to the issue #30