upgrade xgboost-sys

[1a1a11a]

Hi there,
Thank you for your great work! Is it possible to upgrade xgboost-sys which depends on an old version of clang-sys, and causes conflict with boringssl?

error: multiple packages link to native library `clang`, but a native library can be linked only once

package `clang-sys v0.22.0`
    ... which satisfies dependency `clang-sys = "^0.22.0"` (locked to 0.22.0) of package `bindgen v0.36.1`
    ... which satisfies dependency `bindgen = "^0.36"` (locked to 0.36.1) of package `xgboost-sys v0.1.2`
    ... which satisfies dependency `xgboost-sys = "^0.1.2"` (locked to 0.1.2) of package `xgboost v0.1.4`
    ... which satisfies dependency `xgboost = "^0.1.4"` (locked to 0.1.4) of package ...
...
links to native library `clang`

package `clang-sys v1.3.1`
    ... which satisfies dependency `clang-sys = "^1"` (locked to 1.3.1) of package `bindgen v0.59.2`
    ... which satisfies dependency `bindgen = "^0.59"` (locked to 0.59.2) of package `boring-sys v2.0.0`
    ... which satisfies dependency `boring-sys = ">=1.1.0, <3.0.0"` (locked to 2.0.0) of package `boring v2.0.0`

...