Fix #47 : compilation error on NCCL<2.7

Return an error when trying to run alltoall test when compiled
against NCCL<2.7.

src/alltoall.cu

@@ -64,14 +64,18 @@ testResult_t AlltoAllRunColl(void* sendbuff, void* recvbuff, size_t count, ncclD
   size_t rankOffset = count * wordSize(type);
   if (count == 0) return testSuccess;
 
+#if NCCL_MAJOR < 2 || NCCL_MINOR < 7
+  printf("NCCL 2.7 or later is needed for alltoall. This test was compiled with %d.%d.\n", NCCL_MAJOR, NCCL_MINOR);
+  return testNcclError;
+#else
   NCCLCHECK(ncclGroupStart());
   for (int r=0; r<nRanks; r++) {
     NCCLCHECK(ncclSend(((char*)sendbuff)+r*rankOffset, count, type, r, comm, stream));
     NCCLCHECK(ncclRecv(((char*)recvbuff)+r*rankOffset, count, type, r, comm, stream));
   }
   NCCLCHECK(ncclGroupEnd());
-
   return testSuccess;
+#endif
 }
 
 struct testColl alltoAllTest = {