[Bugfix][CI] force download data everything (#208)

* poke ci

* poke again

* force download data everytime

Jenkinsfile

@@ -11,6 +11,7 @@ def setup() {
 
 def build_dgl() {
   sh "if [ -d build ]; then rm -rf build; fi; mkdir build"
+  sh "rm -rf _download"
   dir ("build") {
     sh "cmake .."
     sh "make -j4"