set up multi-os testing in travis

Requires beta access, thankfully the travis gods have blessed us for the
main repo and my fork!

This should allow us to make sure scm_breeze operates reliably in both
Linux and BSD/Darwin, because there are small shell differences
(especially with default tools) that are causing errors I noticed on
MacOSX.

.travis.yml

@@ -1,3 +1,9 @@
-script: ./run_tests.sh
-before_script:
-  - sudo apt-get install zsh
+os:
+  - linux
+  - osx
+
+install:
+  - ./test/support/travisci_deps.sh
+
+script:
+  - ./run_tests.sh