Use sudo for linux travis build

Travis build failed because of memory limit.

Author: Seulgi Kim

.travis.yml

@@ -12,7 +12,8 @@ script:
 matrix:
  allow_failures:
  - rust: nightly
+ include:
+ - os: linux
+ sudo: required
+ - os: osx
 cache: cargo
-os:
- - linux
- - osx