test/spec_helper: fix needs_svn.
test/spec_helper: fix needs_svn.

Use the shim to find a valid Subversion.

MikeMcQuaid/brew,JCount/brew,nandub/brew,Homebrew/brew,Homebrew/brew,Homebrew/brew,EricFromCanada/brew,reitermarkus/brew,konqui/brew,reitermarkus/brew,vitorgalvao/brew,JCount/brew,konqui/brew,mahori/brew,EricFromCanada/brew,sjackman/homebrew,JCount/brew,JCount/brew,EricFromCanada/brew,nandub/brew,claui/brew,claui/brew,reitermarkus/brew,vitorgalvao/brew,reitermarkus/brew,EricFromCanada/brew,konqui/brew,mahori/brew,claui/brew,Homebrew/brew,vitorgalvao/brew,nandub/brew,vitorgalvao/brew,nandub/brew,sjackman/homebrew,mahori/brew,mahori/brew,claui/brew,sjackman/homebrew,MikeMcQuaid/brew,konqui/brew,sjackman/homebrew,MikeMcQuaid/brew,MikeMcQuaid/brew