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.