Update travis for Go 1.15 (pquerna#58)

Merge pull request pquerna#42 from pquerna/pq/expose_setting_secret

Allow calls to {htop,totp}.Generate() to specify the secret

Merge pull request pquerna#31 from champkeh/hotp-digits-check

check hotp key generation opts.Digits

Merge pull request pquerna#19 from pquerna/pq/update_golang_versions

[CI] Update Golang versions on Travis