update travis config
update travis config

- use build matrix to test against multiple versions of WP (including trunk), not just latest stable.
- remove WP_MULTISITE=0 for now
- split up configuration into its appropriate build lifecycle steps, where applicable.
- move notification config to bottom
- change deploy config to master only, on 5.6, wp latest
- setup caching for composer vendor directory

aaemnnosttv/silk,aaemnnosttv/silk