Make Travis test alacritty on nightly Rust, but allow failures

This allows us to notice and report any regressions that have occured on
nightly, without requiring tests to pass on nightly for the entire build
to succeed.

The `fast_finish` flag will cause Travis to mark the build as successful
if the only builds still running are allowed to fail (e.g. `nightly`).
This allows us to manually inspect builds (or implement some form of
notifications) to see if any nightly regressions have occured, without
slowing down the overall build time.

.travis.yml

@@ -10,6 +10,12 @@ os:
 rust:
   - 1.15.0
   - stable
+  - nightly
+
+matrix:
+  fast_finish: true
+  allow_failures:
+    - rust: nightly
 
 script:
   - cargo test --no-default-features