Unpin cpplint

https://github.com/dmlc/treelite/pull/587 introduced the package pin `cpplint==1.6.1` to work around the bug in https://github.com/cpplint/cpplint/issues/293. Once https://github.com/cpplint/cpplint/issues/293 is fixed and new version of cpplint is release, the pin should be removed.