ruby: 2.5.5 -> 2.6.3

According to nix-review, this does not introduce any new build failures and I
had it run on my systems for a while, but there might be breakages at runtime.

Since I'm not going to go through every Ruby package one at a time, I
think the best thing to do is bump this so we can get bug reports of
breakages from people who actually use those packages.

pkgs/top-level/all-packages.nix

@@ -8942,7 +8942,7 @@ in
     ruby_2_5
     ruby_2_6;
 
-  ruby = ruby_2_5;
+  ruby = ruby_2_6;
 
   mruby = callPackage ../development/compilers/mruby { };