Removing 'ruby-' prefix per convention

I received the following warning from rbenv when cloning the project :
```
warning: ignoring extraneous `ruby-' prefix in version `ruby-2.0.0-p247'
         (set by ~/dev/personal/sendgrid/docs/.ruby-version)
```
It looks like the common convention for `.ruby-version` files is to drop the `ruby-` prefix.

See : [A Common .ruby-version File For Ruby Projects](https://gist.github.com/fnichol/1912050) for more on this issue.