Vim 8.1 release

Update version number and information.  Fix a couple of tests.
diff --git a/runtime/doc/develop.txt b/runtime/doc/develop.txt
index 28ba7cd..20a623f 100644
--- a/runtime/doc/develop.txt
+++ b/runtime/doc/develop.txt
@@ -1,4 +1,4 @@
-*develop.txt*   For Vim version 8.0.  Last change: 2018 May 02
+*develop.txt*   For Vim version 8.1.  Last change: 2018 May 02
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar