Better travis workflow
Better travis workflow

- Added composer caching
- use before_install to update composer and add the bookdown2mkdocs dep
- use install to do the actual `composer install` (wrapping it in a
  travis_retry); report what was installed on completion (since caching may
  remove the need to install anything)

nightlinus/zend-expressive,pborreli/zend-expressive,matwright/zend-expressive,grizzm0/zend-expressive,jguittard/zend-expressive,prolic/zend-expressive,ezimuel/zend-expressive,weierophinney/zf-docs-toolchain-experiment,moderndeveloperllc/zend-expressive,jguittard/zend-expressive,prolic/zend-expressive,kynx/zend-expressive,kynx/zend-expressive,kynx/zend-expressive,matwright/zend-expressive,jguittard/zend-expressive,grizzm0/zend-expressive,matwright/zend-expressive,kingjerod/zend-expressive,pborreli/zend-expressive,grizzm0/zend-expressive,acelaya/zend-expressive,zf2timo/zend-expressive,pborreli/zend-expressive,prolic/zend-expressive,kingjerod/zend-expressive