Modify Linux travis config to run content and cpu ref tests.
Add support for showing debug borders

This can help debug issues with the compositor and show when a page is
creating layers.
Merge pull request #2912 from mrobinson/debug-borders

Add support for showing debug borders
Merge pull request #2904 from glennw/linux-ref-test

Modify Linux travis config to run content and cpu ref tests.
Add explicit cast from size_t to u64.

Fixes compile error on non-64-bit targets.
Fix compile error and warning on Android
Update to latest string-cache.

Includes fix for servo/string-cache#4.
Exclude host libs from Android package.

Fixes startup error caused by packaging the wrong libphf.so.