Update to Rails v4.1.8

Rails 4.1.8 is quite mature now. With 4.2.0 coming out soon, it's a good time to update.
* Updated the thinking-sphinx gem also, for compatibility with Rails 4.1.x (tested and seems to work fine).
* No other gems were updated, aside from rails, thinking-sphinx and their dependencies.
* I've updated the config files to closely match the new Rails defaults. Should behave the same as before.
* The new Rails-recommended way to set secret_key_base is via the SECRET_KEY_BASE environment variable read by secrets.yml, but setting it in config/initializers/secret_token.rb as per the Lobsters readme still works fine, as a fallback.
* Slightly simplified readme instructions for creating initial user and tag.