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

test/support/travisci_deps.sh

@@ -0,0 +1,14 @@
+#!/usr/bin/env bash
+# Installs dependencies for travis-ci environments.
+
+# Install dependencies, which looks to be just bash & zsh.
+#
+# Darwin has zsh preinstalled already, so only need to install on Ubuntu.
+#
+# Note: $TRAVIS_OS_NAME will only be set on text boxes with multi-os enabled,
+# so use negation test so it will fail gracefully on normal Travis linux setup.
+#
+# TODO: also perhaps later only on ZSH test box if we split those
+if [[ "$TRAVIS_OS_NAME" != "osx" ]]; then
+  sudo apt-get install zsh
+fi