Update wx version in build instructions

linux/build.txt

@@ -36,7 +36,7 @@ sudo apt-get install git git-gui gitk
 
 wxWidgets:
 
- 1) Clone wxWidgets and checkout 3.1.1 from the Audacity fork of the 
+ 1) Clone wxWidgets and checkout 3.1.3 from the Audacity fork of the 
     wxWidgets project: 
        https://github.com/audacity/wxWidgets/....
 
@@ -52,7 +52,7 @@ wxWidgets:
       git submodule update --init
 
   2) Follow instructions for building at 
-    https://github.com/audacity/wxWidgets/blob/v3.1.1/docs/gtk/install.txt
+    https://github.com/audacity/wxWidgets/blob/v3.1.3/docs/gtk/install.txt
 
     So...
 

mac/cmake_build.txt

@@ -17,7 +17,7 @@ release. Still, read this document throughly before starting.
 
       git clone https://github.com/audacity/audacity
 
-3) Clone wxWidgets 3.1.1:
+3) Clone wxWidgets 3.1.3:
 
       git clone --recurse-submodules https://github.com/audacity/wxWidgets.git
 
@@ -29,20 +29,20 @@ release. Still, read this document throughly before starting.
    such as Homebrew because this will cause problems.
 
    We have applied some Audacity specific patches to wxWidgets to better 
-   support VoiceOver, so please use the audacity-fixes-3.1.1 branch. It's
+   support VoiceOver, so please use the audacity-fixes-3.1.3 branch. It's
    the default branch on our GitHub copy:
 
-      https://github.com/audacity/wxWidgets/tree/audacity-fixes-3.1.1
+      https://github.com/audacity/wxWidgets/tree/audacity-fixes-3.1.3
 
    You can be certain the correct branch is active with:
 
-      git checkout audacity-fixes-3.1.1
+      git checkout audacity-fixes-3.1.3
 
 4) Change directory to the folder where wxWidgets was cloned and build it using:
 
       sudo <path to Audacity source>/mac/scripts/build_wxwidgets
 
-5) The config command should return "3.1.1" if the install was successful:
+5) The config command should return "3.1.3" if the install was successful:
 
       /usr/local/x86_64/bin/wx-config --version