do not run benchmarks

They take forever, and nobody ever looks at the output anyway. Just
making sure they compile is enough.
diff --git a/.travis.yml b/.travis.yml
index 4c52744..b8bb203 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -12,10 +12,10 @@
   - if [ "$TRAVIS_RUST_VERSION" = "nightly" ]; then
       cargo test --verbose --features i128;
       cargo test --verbose --no-default-features --features i128 --lib;
-      cargo bench --verbose;
-      cargo bench --verbose --no-default-features;
-      cargo bench --verbose --features i128;
-      cargo bench --verbose --no-default-features --features i128;
+      cargo bench --verbose --no-run;
+      cargo bench --verbose --no-run --no-default-features;
+      cargo bench --verbose --no-run --features i128;
+      cargo bench --verbose --no-run --no-default-features --features i128;
     fi
 branches:
   only: