Clarify how escaping works for encrypting env vars

Fixes #389
Add Atom feed blog post
Correct word usage
Add pictures for Atom feed blog post
Fix link for Travis CI
Trim the picture
Merge pull request #423 from travis-ci/ha-blog-atom_feed

Atom feed blog post
Fix our product name
Break up curl command into two lines
Note that caching is not available on Mac VMs
Blog post on caching dependencies.
Fix formatting in code blocks.
Merge pull request #425 from travis-ci/mm-caching-blog-post

Blog post on caching dependencies.
Lmao if I know how YAML works.
Lmao grammar.
Fix assets example.
Fix blog link.
Update build environment for latest changes.
Update imprint.
Merge pull request #418 from travis-ci/hh-clarify-escaping-for-secure-env-vars

Clarify how escaping works for encrypting env vars