Fix DEBIAN_FRONTEND environment variable

As written it almost certainly won't actually do anything since sudo
clears most of the environment.

.travis.yml

@@ -2,8 +2,6 @@ language: python
 sudo: required
 
 env:
-   global:
-      - DEBIAN_FRONTEND=noninteractive
    matrix:
       - BUILD_TYPE=make CC=gcc
       - BUILD_TYPE=make CC=clang
@@ -14,7 +12,7 @@ env:
 
 install:
    - sudo apt-get update
-   - sudo apt-get install -y build-essential devscripts
+   - sudo DEBIAN_FRONTEND=noninteractive apt-get install -y build-essential devscripts
 
 script:
    - 'if [ "$BUILD_TYPE" == "make" ]; then make build && ./test ./dumb-init; fi'