This appears to be a problem with your connection to github and has nothing to do with CUDPP. Appended is my trace from running the same command.
JDO
$ git clone --recursive
https://github.com/cudpp/cudpp.git
Cloning into 'cudpp'...
remote: Counting objects: 8587, done.
remote: Total 8587 (delta 0), reused 0 (delta 0), pack-reused 8587
Receiving objects: 100% (8587/8587), 111.18 MiB | 10.86 MiB/s, done.
Cloning into '/private/tmp/cudpp/ext/cub'...
remote: Counting objects: 26314, done.
remote: Total 26314 (delta 0), reused 0 (delta 0), pack-reused 26314
Receiving objects: 100% (26314/26314), 142.14 MiB | 11.10 MiB/s, done.
Resolving deltas: 100% (23717/23717), done.
Checking connectivity... done.
Submodule path 'ext/cub': checked out 'f0b7d80a39a5837454bdb17f396f3fe833009781'
Cloning into '/private/tmp/cudpp/ext/moderngpu'...
remote: Counting objects: 1244, done.
remote: Total 1244 (delta 0), reused 0 (delta 0), pack-reused 1244
Receiving objects: 100% (1244/1244), 6.72 MiB | 3.23 MiB/s, done.
Resolving deltas: 100% (613/613), done.
Checking connectivity... done.
Submodule path 'ext/moderngpu': checked out 'b6b3ed52f8f7478b969e05242980c97ff21a6ab0'
$