fetchbower: handle packages with slashes in their name

Packages from github repos have slashes in their name. Nix store names
shouldn't have slashes.

Fixes rvl/bower2nix#13

pkgs/build-support/fetchbower/default.nix

···
       ver = if builtins.length components == 1 then version else hash;
     in ver;
 
+  bowerName = name: lib.replaceStrings ["/"] ["-"] name;
+
   fetchbower = name: version: target: outputHash: stdenv.mkDerivation {
-    name = "${name}-${bowerVersion version}";
+    name = "${bowerName name}-${bowerVersion version}";
     SSL_CERT_FILE = "${cacert}/etc/ssl/certs/ca-bundle.crt";
     buildCommand = ''
       fetch-bower --quiet --out=$PWD/out "${name}" "${target}" "${version}"