Updates to travis configuration file

- Turned apt caching on in travis builds
- Configured to use copy of composer independent from build server
	This is done to overcome issues with composer self-update failing on connection to the composer site, and avoid the time needed to self-update on every build since we'll have the latest version cached
- Added call to composer --version so we will easily know what version is used (since default is not being used, travis call to this is inaccurate)