Re-engage servo-linux-cross1 on Travis

Upstream has released new packages for Ubuntu Trusty which resolve
the installation conflicts encountered when installing the ARM g++ packages.

See https://bugs.launchpad.net/ubuntu/+source/gcc-4.8-armhf-cross/+bug/1557205/comments/16
for a comment confirming release of the upstream fix.

This reverts commit aa84355.

.travis.yml

@@ -26,8 +26,6 @@ matrix:
       dist: trusty
       language: python
       python: 3.5
-  allow_failures:
-    - env: SALT_NODE_ID=servo-linux-cross1
 
 script: .travis/dispatch.sh