.travis.yml: Add sentinel job
.travis.yml: Add sentinel job

This makes Python 3.6 the sentinel job,
so it runs by itself initially and causes
all other jobs to be cancelled if it fails.

SanketDG/coala,jayvdb/coala,SanketDG/coala,coala-analyzer/coala,jayvdb/coala,coala/coala,coala-analyzer/coala,SanketDG/coala,coala/coala,jayvdb/coala,coala/coala,coala-analyzer/coala