deploy: Add `true` as a valid key instead of `:on`

The YAML 'on: foo' is parsed into the Ruby hash '{true=>"foo"}'. We
call 'deep_symbolize_keys' on the hash, but boolean values are not
symbolized.
Merge pull request #175 from travis-ci/hh-fix-deploy-on

deploy: Add `true` as a valid key instead of `:on`