In travis don't treat dmd-nightly failures as blocking errors

https://issues.dlang.org/show_bug.cgi?id=17981

Author: n8sh

.travis.yml

@@ -20,6 +20,8 @@ matrix:
     - {os: linux, d: ldc, env: ARCH="x86", addons: {apt: {packages: [[gcc-multilib]]}}}
     - {os: linux, d: dmd-beta, env: ARCH="x86", addons: {apt: {packages: [[gcc-multilib]]}}}
     - {os: linux, d: dmd, env: ARCH="x86", addons: {apt: {packages: [[gcc-multilib]]}}}
+  allow_failures:
+    - {d: dmd-nightly}
 branches:
   only:
     - master