Switch CLIENT_VERSION_IS_RELEASE to true for v0.15 (#3306)

Author: UdjinM6

configure.ac

@@ -4,7 +4,7 @@ define(_CLIENT_VERSION_MAJOR, 0)
 define(_CLIENT_VERSION_MINOR, 15)
 define(_CLIENT_VERSION_REVISION, 0)
 define(_CLIENT_VERSION_BUILD, 0)
-define(_CLIENT_VERSION_IS_RELEASE, false)
+define(_CLIENT_VERSION_IS_RELEASE, true)
 define(_COPYRIGHT_YEAR, 2020)
 define(_COPYRIGHT_HOLDERS,[The %s developers])
 define(_COPYRIGHT_HOLDERS_SUBSTITUTION,[[Dash Core]])