Expand Travis Support

- Added option to set Environment variables like SYMFONY_ENV and APPLICATION_ENV.
- Disabled composer usage in build.xml when using Travis to speed up travis builds (the travis-shipped composer is being used)
- Conditionally not render `prepare` target (including dependencies) and not enable it in the default build target when no build artifacts are generated
- Conditionally not render `composer-run-install` target (including dependencies) and not enable it in the default build target if Travis support is enabled