.travis.yml: Force apt to behave non-interactively
.travis.yml: Force apt to behave non-interactively

Travis started presenting an apt-get prompt, then timing out after 10
minutes of inactivity. Presumably they've changed their configuration
so we need to specify noninteractive mode ourselves.

LewisHaley/stb-tester,stb-tester/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester