Remove {httr} and replace with already used {curl} dependency

[philipp-baumann]

There is two functions where we do http(s) requests via {httr}

• get_sri_hash_deps()
• get_right_commit()

It's quite straightforward and safe to do this directly with low-level curl::curl_fetch_memory() and appropriate error handle. I can make a PR, so we can get rid of httr and have even less dependencies.

[philipp-baumann]

model it after here: https://cran.r-project.org/web/packages/curl/vignettes/intro.html

[philipp-baumann]

addressing it in #189

[philipp-baumann]

closing, addressed in #189