WIP: Use the credentials package

[stewid]

Work in progress to use the 'credentials' package to setup and retrieve HTTPS and SSH credentials, see #421

[stewid]

@jdblischak I've created a branch, but so far I have just bumped the version

[lintr-bot]

R/checkout.R:160:1: style: functions should have cyclomatic complexity of less than 15, this has 21.

​checkout <- function(object = NULL,
^

R/config.R:56:1: style: functions should have cyclomatic complexity of less than 15, this has 18.

​config <- function(repo = NULL, global = FALSE, user.name, user.email, ...) {
^

R/credential.R:61:1: style: Lines should not be more than 80 characters.

​##' \url{https://help.github.com/articles/creating-an-access-token-for-command-line-use}
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[coveralls]

Coverage decreased (-0.03%) to 81.234% when pulling 1ec5e30 on use-credentials-pkg into 1b1ba56 on master.