Do not link examples on CI

* use cargo test --tests --benches

This does everything cargo test does, except linking the examples.
Thus, we can speed up the testing.
The examples get checked by the later cargo check calls

* Add cargo check without (default) features

This was added to the nightly build only,
because the stable build has more work already.
It helps to detect errors that arise from
missing feature cfgs.

.travis.yml

@@ -60,7 +60,7 @@ before_script:
 script:
 - |
   echo "Build and test"
-  cargo test --all || exit 1
+  cargo test --all --tests --benches --features sdl_controller,profiler,json,saveload  || exit 1
 - |
   if [ ${TRAVIS_RUST_VERSION} == "stable" ] && [ ${TRAVIS_OS_NAME} == "linux" ]
   then
@@ -77,5 +77,7 @@ script:
   if [ ${TRAVIS_RUST_VERSION} == "nightly" ]
   then
     echo "Check common features"
+    cargo check --all --no-default-features
+    cargo check --all
     cargo check --all --all-features
   fi