chore: increase vm.max_map_count

This is mainly used for elasticsearch but adding it to the generic test suite for all datastore plugins makes it easier to copy the travis config to each plugin.

.travis.yml

@@ -4,5 +4,6 @@ env:
   - DOKKU_VERSION=master
   - DOKKU_VERSION=v0.17.0
 install: make setup
+before_script: sudo sysctl -w vm.max_map_count=262144
 script: make test
 after_failure: make report