Release 1.7.1 (dmlc#8413)

* Release 1.7.1

* Review comment

Release 1.7

Remove cleanup script in R package. (dmlc#8370)

Fix typo. (dmlc#8192)

Bump release version to 1.6.1. (dmlc#7872)

Make 1.6.0 release. (dmlc#7813)

Make rc1 release. (dmlc#7764)

Bump release version to 1.5.2. (dmlc#7567)

Bump version to 1.5.1

Release 1.5.0. (dmlc#7317)