Update 2018-12-06-Rust-1.31-and-rust-2018.md

Add something about rustup self update to fix install problems

Author: nrc

posts/2018-12-06-Rust-1.31-and-rust-2018.md

@@ -350,6 +350,9 @@ Work on IDE support is not finished, in particular code completion is not up
 to scratch in the RLS-based editors. However, if you mainly want support for
 types, documentation, and 'go to def', etc. then you should be happy.
 
+If you have probelms installing any of the tools with Rustup, try running
+`rustup self update`, and then try again.
+
 ### Tool lints
 
 In [Rust 1.30](https://blog.rust-lang.org/2018/10/25/Rust-1.30.0.html), we
@@ -467,4 +470,4 @@ release, more so than others, this list does not truly capture the amount of
 work and the number of people who have contributed. Each release is only six
 weeks, but this release is the culmination of three years of effort, in
 countless repositories, by numerous people. It's been a pleasure to work with
-you all, and we look forward to continuing to grow in the next three years.
+you all, and we look forward to continuing to grow in the next three years.