Pin to particular installer version

[shannah]

Currently the download page on jdeploy.com always uses the latest version of the jdeploy installer to install the apps. In general this is ideal as this makes sure that app owners don't need to republish their apps to benefit from bug-fixes in the installer. However, it may be desirable to pin to a specific version of the installer so that they aren't "surprised" by changes to the way the installer works.

This ability is already built into the app.xml format, but it isn't documented, nor is it connected (I think) to the package.json file yet. Need to add a flag in the GUI and in the package.json file to specify the installer version. E.g.

"installerVersion" : "3.1.9"
or
"installerVersion":"LATEST",

or any other valid semantic version identifier.