#346 Switch from YAML for TOML

@ali-graham, @jessereynolds  
Changes for #346 Switch from YAML for TOML. Specs and Features running green, but had to leave support FLAPJACK_ENV in the test harness and `email.rb`.

NB: This doesn't touch on #285 or #618, but sets the groundwork for implementing these.

This PR is pointing back at `master`, as I can't work out which published branch this should target. Any advice appreciated.