[DOCKER] Upgrade ci-cpu to latest v0.50 (apache#2901)

Author: tqchen

Jenkinsfile

@@ -22,7 +22,7 @@
 //
 ci_lint = "tvmai/ci-lint:v0.50"
 ci_gpu = "tvmai/ci-gpu:v0.51"
-ci_cpu = "tvmai/ci-cpu:v0.41"
+ci_cpu = "tvmai/ci-cpu:v0.50"
 ci_i386 = "tvmai/ci-i386:v0.50"
 
 // tvm libraries