Add support for `sprockets-rails` 3 and `sprockets` 4.
Add support for `sprockets-rails` 3 and `sprockets` 4.

`sprockets-rails` 3 already precompiles any asset that is referenced by the `font-path`
and `image-path` helpers, and `sprockets` 4 does not support Regexpes as the
`precompile` flag.

sonipalak/demo,CapeSepias/bootstrap-sass,JasonRivers/bootstrap-sass,mosandquery/bootstrap-sass,twbs/bootstrap-sass,rightscale-design/bootstrap-sass,bahrul221/bootstrap-sass,beauxjames/bootstrap-sass,smartexpose/twbs-bootstrap-sass,zBMNForks/bootstrap-sass,Wcc723/capstrap-open,karlitxo/bootstrap-sass,twbs/bootstrap-sass,karlitxo/bootstrap-sass,bahrul221/bootstrap-sass,dWhisper/bootstrap-sass,cgvarela/bootstrap-sass,JasonRivers/bootstrap-sass,karlitxo/bootstrap-sass,zBMNForks/bootstrap-sass,kyled09/splashPage,orangevka/aori,SSLcom/Bootsharp,sonipalak/demo,rightscale-design/bootstrap-sass,twbs/bootstrap-sass,beauxjames/bootstrap-sass,thatsus/bs-sass,CapeSepias/bootstrap-sass,mosandquery/bootstrap-sass,cgvarela/bootstrap-sass,Wcc723/capstrap-open,kyled09/splashPage,orangevka/aori,beauxjames/bootstrap-sass,twbs/bootstrap-sass,JasonRivers/bootstrap-sass,rightscale-design/bootstrap-sass,zBMNForks/bootstrap-sass,SSLcom/Bootsharp,Wcc723/capstrap-open,sonipalak/demo,cgvarela/bootstrap-sass,SSLcom/Bootsharp,dWhisper/bootstrap-sass,bahrul221/bootstrap-sass,CapeSepias/bootstrap-sass,Wcc723/capstrap-open,dWhisper/bootstrap-sass,CapeSepias/bootstrap-sass,ruiii/bootstrap-sass,cgvarela/bootstrap-sass,bahrul221/bootstrap-sass,beauxjames/bootstrap-sass,rightscale-design/bootstrap-sass,karlitxo/bootstrap-sass,thatsus/bs-sass,mosandquery/bootstrap-sass,dWhisper/bootstrap-sass,zBMNForks/bootstrap-sass,kyled09/splashPage,ruiii/bootstrap-sass,thatsus/bs-sass,smartexpose/twbs-bootstrap-sass,orangevka/aori,SSLcom/Bootsharp,ruiii/bootstrap-sass,JasonRivers/bootstrap-sass,smartexpose/twbs-bootstrap-sass,mosandquery/bootstrap-sass,sonipalak/demo,smartexpose/twbs-bootstrap-sass,thatsus/bs-sass,ruiii/bootstrap-sass,kyled09/splashPage,orangevka/aori