v0.4.0

Changelog

f9ecd96 Merge pull request #10 from citymapper/fixes
49170c8 Merge pull request #12 from citymapper/allow_env_credentials
96a3199 Merge pull request #13 from citymapper/log_if_listdir_fails
3e226f5 Merge pull request #14 from citymapper/use_go_modules
2214794 Merge pull request #15 from citymapper/upgrade_to_circleci_2_0
022fbb4 Merge pull request #18 from citymapper/use_goreleaser