httr depende do openssl e de curl pacote.
O pacote openssl precisa do requisito do sistema libssl-dev
------------------------- ANTICONF ERROR ---------------------------
Configuration failed because openssl was not found. Try installing:
* deb: libssl-dev (Debian, Ubuntu, etc)
...
O pacote curl precisa do requisito do sistema libcurl4-openssl-dev :
------------------------- ANTICONF ERROR ---------------------------
Configuration failed because libcurl was not found. Try installing:
* deb: libcurl4-openssl-dev (Debian, Ubuntu, etc)
...
Então, para instalar, você precisará executar:
sudo apt-get install libssl-dev libcurl4-openssl-dev
depois, install.packages("plotly") deve funcionar.