Install fech-source over https

[zstumgoren]

Fech currently installs the fech-source submodule (specified in .gitmodules) using the git/ssh strategy. This causes a source-based install of Fech -- via bundler in our case -- to fail for a user that doesn't have an ssh public key on Github.

Here's an example gist demonstrating the issue.

This likely won't be a problem for local development by most contributors, who presumably have a Github account with their ssh key.

But it proved problematic for us in a production context, where we're installing Fech from source while we await release of a new Gem version that contains the FEC v8.2 fix.

Updating .gitmodules to use https for fech-source resolves the issue.

PR to follow.