bitbucket_remote should have a default host

[antoine-sachet]

Just like github_remote and gitlab_remote, bitbucket_remote should specify the default host.
Otherwise, remote bitbucket dependencies cannot be installed even if #337 is fixed.

The fix is in install-bitbucket.R, changing this:

bitbucket_remote <- function(repo, ref = "master", subdir = NULL,
                              auth_user = NULL, password = NULL, sha = NULL,
                              host = NULL, ...) { .. }

to this:

bitbucket_remote <- function(repo, ref = "master", subdir = NULL,
                              auth_user = NULL, password = NULL, sha = NULL,
                              host = "api.bitbucket.org/2.0", ...) { .. }