Update CI to test const-generics feature

Author: survived

.travis.yml

@@ -27,7 +27,9 @@ script:
     cargo test --no-default-features --features std &&
     cargo test --no-default-features --features "std i128" &&
     cargo test --no-default-features --features "std core_hint_black_box" &&
-    cargo test --no-default-features --features "std i128 core_hint_black_box"
+    cargo test --no-default-features --features "std const-generics" &&
+    cargo test --no-default-features --features "std i128 core_hint_black_box" &&
+    cargo test --no-default-features --features "std i128 core_hint_black_box const-generics"
 
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