* Retry downloading of depends sources 3 times
* Manually invoke apt-get update and install instead of using addon
This allows us to use travis_retry on these operations.
* Add exception for depends/Makefile in .gitignore
* Remove --retry from curl call
This avoids future confusion as we don't rely on curl anymore to properly
retry downloads. curls does only retry on transient errors, while Travis
might cause all kinds of errors which circumvent the retry logic of curl.
