.travis.yml: Pre-install apt packages
.travis.yml: Pre-install apt packages

These packages are in the travis package whitelist,
and can be moved to the initial worker initialisation.

Closes https://github.com/coala/coala-bears/issues/2539

coala-analyzer/coala-bears,coala/coala-bears,coala/coala-bears,refeed/coala-bears,coala-analyzer/coala-bears,coala-analyzer/coala-bears,refeed/coala-bears,coala-analyzer/coala-bears,coala/coala-bears,coala/coala-bears,refeed/coala-bears,coala/coala-bears,coala/coala-bears,refeed/coala-bears,coala-analyzer/coala-bears,coala/coala-bears,coala/coala-bears,coala/coala-bears,coala-analyzer/coala-bears,coala/coala-bears,refeed/coala-bears,coala-analyzer/coala-bears,coala-analyzer/coala-bears,coala/coala-bears,coala-analyzer/coala-bears,refeed/coala-bears,refeed/coala-bears,refeed/coala-bears,refeed/coala-bears,coala/coala-bears,refeed/coala-bears,refeed/coala-bears