Prevent constant ElasticAPM warnings in development by disabling it

No changes are needed in prod, since the `enabled` setting defaults
to "true", and prod obviously isn't setting this ENV.

.env.vagrant

@@ -4,3 +4,4 @@ DB_HOST=/var/run/postgresql/
 DB_MASTER_URL=postgres:///gabsocial_development
 DB_SLAVE1_URL=postgres:///gabsocial_development
 DB_SLAVE2_URL=postgres:///gabsocial_development
+ELASTIC_APM_ENABLED=false

config/elastic_apm.yml

@@ -9,3 +9,5 @@ secret_token: <%= ENV['ELASTIC_APM_SECRET_TOKEN'] || '' %>
 
 # Set custom APM Server URL (default: http://localhost:8200)
 server_url:  <%= ENV['ELASTIC_APM_URL'] || 'http://localhost:8200'%>
+
+enabled: <%= ENV['ELASTIC_APM_ENABLED'] || 'true' %>