WIP: Server plugins via rlua (Lua 5.3) (#125)

* Add 'plugins' field to project and add rlua

* Scaffold out new SnapshotContext type (again) with plugin state

* Almost functional snapshot system with rlua proof-of-concept

* Gate plugin config on 'plugins-enabled' feature, tell Travis to test all features

* Guard remaining plugin setup code behind feature

* Bump minimum version to 1.33, should've caught this before

* Whoops, latest Rust is 1.32, not 1.33

server/tests/read_projects.rs

@@ -86,6 +86,7 @@ fn single_partition_game() {
         Project {
             name: "single-sync-point".to_string(),
             tree: root_node,
+            plugins: Vec::new(),
             serve_port: None,
             serve_place_ids: None,
             file_location: project_location.join("default.project.json"),