Update elastic_stack repository URL

The repository was renamed to match the module name.  Use the new URL.

Author: smortex

.fixtures.yml

@@ -5,7 +5,7 @@ fixtures:
     augeas_core: https://github.com/puppetlabs/puppetlabs-augeas_core.git
     concat: https://github.com/puppetlabs/puppetlabs-concat.git
     datacat: https://github.com/richardc/puppet-datacat.git
-    elastic_stack: https://github.com/voxpupuli/puppet-elastic-stack.git
+    elastic_stack: https://github.com/voxpupuli/puppet-elastic_stack.git
     java: https://github.com/puppetlabs/puppetlabs-java.git
     java_ks: https://github.com/puppetlabs/puppetlabs-java_ks.git
     stdlib: https://github.com/puppetlabs/puppetlabs-stdlib.git