Remove .ruby-version file and add note to README regarding minimum Ruby version.

@imathis - look ok?

Closes #1365.
".git" added to sample string with SSH repository URL

Octopress does not work with a repo URL like a "git@github.com:your_username/your_username.github.io" ("ERROR: repository not found" error appears).
It is necessary to add ".git" to the end of this line.
Merge pull request #1367 from nagimov/patch-1

".git" added to sample string with SSH repository URL
Merge https://github.com/imathis/octopress into source