Fixed typo

Author: gigi81

build.bat

@@ -12,7 +12,7 @@ SET KIBANA_VERSION=4.5.0
 
 rem ---------- installer version
 SET INSTALLER_VERSION=1.0.0
-IF DEFINED APPVEYOR_BUILD_VERSION SET INSTALLER_VERSION=APPVEYOR_BUILD_VERSION
+IF DEFINED APPVEYOR_BUILD_VERSION SET INSTALLER_VERSION=%APPVEYOR_BUILD_VERSION%
 echo "Building installer v%INSTALLER_VERSION%"
 
 rem ---------- Download packages ----------