feat/test Enabling travis-ci cache for node_modules
feat/test Enabling travis-ci cache for node_modules

This commit enables caching on Travis for the node_modules directory,
cf https://docs.travis-ci.com/user/caching/#Caching-directories-%28Bundler%2C-dependencies%29

close #1600

fbasso/ariatemplates,benouat/ariatemplates,simonarbuckle/ariatemplates,ariatemplates/ariatemplates,simonarbuckle/ariatemplates,vcarle/ariatemplates,ymeine/ariatemplates,ariatemplates/ariatemplates,simonarbuckle/ariatemplates,ariatemplates/ariatemplates,divdavem/ariatemplates,ymeine/ariatemplates,divdavem/ariatemplates,fbasso/ariatemplates,vcarle/ariatemplates,ymeine/ariatemplates,benouat/ariatemplates,benouat/ariatemplates,fbasso/ariatemplates,divdavem/ariatemplates,vcarle/ariatemplates