New functions get_github_key() and
get_travis_key() to retrieve public keys from GitHub and
Travis (#23).
add_travis_user() works correctly again, now it uses
the Travis V2 API at https://api.travis-ci.com.
add_github_user() now works correctly when not the
first key is added (#21, @jiwalker-usgs).
get_secret() is now faster, because it uses
fingerprint matching instead of trying to decrypt every file (#19, @jiwalker-usgs).
list_secrets() now works if vault is
not specified (#22, @AlexAxthelm).
First public release.