Update gemfiles to current patch versions; remove 3.0 and add 4.0

Rails 3.0 is no longer receiving bugfixes and has some issues with
saving associations. No longer supported.
Spruce up gemspec

Remove nasty git globbing etc.
Tidy spec helper

Remove generated comments.
Rename #hide and #show to #hide! and #unhide!

#show was too general and felt like it might clash with users' methods.
Bang methods feel good.
Rename to plural

Makes more sense.
Remove some bad methods

Simple code is good code! Past me had made some mistakes.
Move towards more sane use of modules

There was previously too much separation for a task this simple. Also, extending
ActiveRecord::Base by default isn't great. Users must now extend
with Hideable::ActiveRecord manually.
Do away with Core module

Pointlessly named module. It's now called InstanceMethods.
Rename .visible to .not_hidden

Lower potential seep into the user's domain language.
Update README
Update travis conf for Rails 4 and Ruby 2
Twitching double quote audit