travis: test with stable and beta, but not nightly

.travis.yml

@@ -1,7 +1,7 @@
 language: rust
 rust:
+  - stable
   - beta
-  - nightly
 services:
   - redis-server
 env:

src/main.rs

@@ -14,7 +14,7 @@ use spaceapi_server::SpaceapiServer;
 use spaceapi_server::datastore::DataStore;
 use spaceapi_server::redis_store::RedisStore;
 
-
+#[cfg_attr(test, allow(dead_code))]
 fn main() {
     let host = Ipv4Addr::new(127, 0, 0, 1);