Use TRAVIS_REPO_SLUG instead of hard-coding pytest-dev/pytest
Use TRAVIS_REPO_SLUG instead of hard-coding pytest-dev/pytest

I was doing final tests on the script today, and forgot to change
the hardecoded "pytest-dev/pytest", which ended up publishing a
`4.99.10` release to the main repository by mistake, as my token
has access to both my fork and main repository.

I deleted the tag immeditely just a few seconds later, so hopefully
this won't cause major problems.

This change makes it safer to test this in the future, never publishing
to the main repository by mistake (as long as the tags are pushed to the
right repositories of course).

alfredodeza/pytest,RonnyPfannschmidt/pytest,Akasurde/pytest,tomviner/pytest,The-Compiler/pytest,The-Compiler/pytest,nicoddemus/pytest,markshao/pytest,nicoddemus/pytest,tomviner/pytest,pytest-dev/pytest