Encrypt (with jsencrypt) the variable name and value in your browser using the Travis-CI public key of the given repository
Same functionality of the `travis encrypt` tool
Then add it to .travis.yml, in env: global:, so that it will be decrypted by Travis at build time, see: http://docs.travis-ci.com/user/encryption-keys/