Removed forced lower case from VALUE in getPropertiesValue

This makes using custom mscs-xxx-xxx properties with replacement
variables like $SERVER_VERSION possible.

Currently if you set mscs-server-jar to something else than default and include variables like this:
<pre>mscs-server-jar=minecraftforge-installer-$SERVER_VERSION.jar</pre>
it gets read as minecraftforge-installer-$server_version.jar thus preventing the replacement with the correct $SERVER_VERSION set in 'mscs-server-version'

This commit fixes the issue by dropping the forced lower case as mentioned in the commit message.