Bump rc-slider minor version

This is so we can use the `visible` prop
in `tipProps`, introduced in `rc-slider` 8.6.1
(undocumented in the `rc-slider` repo's HISTORY.md)

See react-component/slider#383

Author: wbrgss

package.json

@@ -36,7 +36,7 @@
     "moment": "^2.20.1",
     "prop-types": "^15.6.0",
     "ramda": "^0.24.1",
-    "rc-slider": "^8.3.1",
+    "rc-slider": "^8.6.11",
     "react-addons-shallow-compare": "^15.6.0",
     "react-dates": "^20.1.0",
     "react-docgen": "^3.0.0",