AppVeyor fix: remove existing clones of the java models library.

This commit changes the AppVeyor configuration file such that
the java models library is freshly downloaded for every build. This
prevents JBMC from being called with an out-of-sync version of the
models library, resulting in non-deterministic behaviour.

Author: Degiorgio

appveyor.yml

@@ -18,6 +18,7 @@ install:
     Move-Item win_flex.exe bin\flex.exe -force
     Move-Item FlexLexer.h include\FlexLexer.h -force
     Remove-Item bin\data -Force -Recurse -ErrorAction SilentlyContinue
+    Remove-Item java-models-library-master -Force -Recurse -ErrorAction SilentlyContinue
     Move-Item data bin\data -force
     bison -V
     flex -V