Hashicorp Vault Monitor

BUG FIXES:

• Send all the messages to stdout when the Nagios outputter (-output=nagios) is selected.
  This is required because, as pointed out by unix196, Nagios shows an empty output in case of warning and error messages sent to stderr (if the stderr is not redirected to stdout).

CHANGES:

• When the Nagios outputter is selected (-output=nagios), the messages are now printed without any color.

• Documentation updates.

IMPROVEMENTS:

• hashicorp-vault-monitor now uses Go's official dependency management system, Go Modules, to manage dependencies.

• Include the error message in output when reading the environment variables.
  This will help debug the issues related to environment variables loading.
  Pull Request by maxadamo. Thanks!

• Travis CI now uses go 1.14.x as build target.

• Add CircleCI and SemaphoreCI continuous integration configurations with go 1.13.x and 1.14.x as build targets.