tests: first pass at using global PHPUnit and Behat
travis: install Behat in the local build dir
replace PHPUnit assertEquals() with homegrown variant
replace assertStringMatchesFormat() with assertNumeric()
replace remaining assertion calls with direct exceptions
fix assertNumeric() usage
update utils/dev-build
Merge pull request #665 from wp-cli/no-dev-deps

Can't run Behat while having community packages installed