add a `build:mri` task to build all the CRubies
add a `build:mri` task to build all the CRubies

rake-compiler/rake-compiler-dock,larskanis/rake-compiler-dock,larskanis/rake-compiler-dock,rake-compiler/rake-compiler-dock,rake-compiler/rake-compiler-dock,rake-compiler/rake-compiler-dock,larskanis/rake-compiler-dock,rake-compiler/rake-compiler-dock