Update last known nightly

.travis.yml

@@ -2,16 +2,20 @@ language: rust
 rust:
   - stable
   - beta
-  - nightly-2016-03-17 # Last known to be working nightly
+  - nightly-2016-04-21 # Last known to be working nightly
   - nightly
 
+matrix:
+  allow_failures:
+    - rust: nightly
+
 addons:
   apt:
     packages:
       - libprotoc-dev
       - portaudio19-dev
       - libpulse-dev 
-      
+
 script:
     - cargo build --no-default-features --features "with-syntex"
     - cargo build --no-default-features --features "with-syntex with-tremor"
@@ -22,7 +26,3 @@ script:
     - if [[ $TRAVIS_RUST_VERSION == *"nightly"* ]]; then
         cargo build --no-default-features;
       fi
-
-matrix:
-  allow_failures:
-    - rust: nightly