Revert "Remove support to Rails 3"

This reverts commit ccdcf4755d3446daaeed4d4fab41a003779d60d2.
Conditional test Rails 3 and Rails 4
Test with Ruby 1.8
Chaging the public path doesn't work on Rails 3.0
Improve CHANGELOG [ci skip]
Release 2.1.1
Instead of complaining, add the dependency automatically

We need to use *both* `depend_on` and `depend_on_asset`, because the
latter (bugfully?) considers changes to the referenced asset's
dependencies, but not the file itself (making it entirely useless for an
image).

In the corresponding test, we call a protected Sprockets method... by
design, this isn't something we're supposed to care about / access. But
actually manipulating assets to ensure the dependency has taken hold
seems rather like overkill.
Merge pull request #126 from matthewd/automatic_dependencies

Instead of complaining, add the dependency automatically
Merge pull request #127 from matthewd/helper_version

Version the gem's helpers; if they change, the cache must go
Conflicts:
	sprockets-rails.gemspec
Setup gem properly
Load Action View so hooks work
Remove unneeded comment
Remove wrong comment
Test with released version
Define railties as development dependency
Add CHANGELOG entry for 2.1.2 [ci skip]
Prepare for the 2.1.2 release
Use string in gemspec instead of using constant

For some reason changing the load path break tests with Rails 3.1 and
Rails 3.2.

I can't understand why, so it is better to remove.
Merge pull request #129 from matthewd/precompile_extensions

Check precompile for the asset's logical_path
Merge pull request #130 from matthewd/assets_initializer

Precompile configuration now lives in an initializer