gradle: Fix explore version

kolibri-explore-plugin passed in the tag rather than the version and
unfortunately, `update-explore-version.sh` doesn't do any validation.
That will be addressed with a followup.

app/gradle.properties

@@ -13,7 +13,7 @@ kolibriUrl=https://github.com/learningequality/kolibri/releases/download/v0.16.0
 
 # kolibri-explore-plugin version or wheel URL. The wheel URL is
 # preferred if both are set.
-exploreVersion=v7.9.0
+exploreVersion=7.9.0
 exploreUrl=
 
 # apps-bundle.zip URL. If not set, the URL is derived from the explore