Auto merge of #3522 - matthiaskrgr:target_dir, r=phansch
Auto merge of #3522 - matthiaskrgr:target_dir, r=phansch

travis: base-tests: share CARGO_TARGET_DIR between check runs of subcrates to avoid unnecessarily recompiling deps.

aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust