ci: don't try to run tests on OSX

We want the package to be able to be in the build tree without failing
OSX but we don't really care about the tests being runnable on this platform.

Signed-off-by: Paul Osborne <[email protected]>

Author: posborne

.travis.yml

@@ -7,4 +7,4 @@ os:
 
 script:
 - cargo build --verbose
-- cargo test --verbose
+- if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then cargo test --verbose; fi