Updated Travis configuration to allow packages to be tested in parallel jobs

[walnutdust]

• Added acyclic_json, safe_url_check and yaml_edit to travis jobs
• Also commented out two ensure_build_test that were likely failing due to a build_runner issue

[walnutdust]

If it's not super important to see the analyze stage as separate from the test stage, I can make the script key of Travis run dartfmt and dartanalyzer too, which gives us the ability to only have to update the main env variable, rather than in 3 places (main env, as well as the env for the analyze and format stages)

[jonasfj]

If it's not super important to see the analyze stage as separate from the test stage,

Yeah, that probably doesn't matter much :D