Just to add a little warning: it might be obvious, but be careful before deciding to disable "Use secure download method for HTTP", since this has implications for the trustworthiness of the downloaded material and the security of your system.
From the RStudio support page about secure package download:
Using an encrypted connection is strongly recommended as it provides much higher assurance that the code you are downloading is in fact from a legitimate CRAN mirror rather than from another server posing as one.