optim(ci): clone bats-core with --depth=1 (#64)

- should be a good bit faster than cloning with the whole history

Author: agilgur5

travis/get_bats-core.sh

@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 
-git clone https://github.com/bats-core/bats-core.git
+git clone https://github.com/bats-core/bats-core.git --depth=1
 cd bats-core
 sudo ./install.sh /usr/local