allow skipping pending migrations using an environment variable

if PENDING_MIGRATIONS_OK is set, the usual pending migration check will not be
performed. this is useful for automation tools (e.g. bots that do deployments).

do not set this variable for your regular deployments unless you are absolutely
sure what you are getting into.
bump version to 0.0.5
add warning message when PENDING_MIGRATIONS_OK and REVERSE_DEPLOY_OK are set

adding logger.warn statement to make sure users don't miss the fact that environment
variables that caused these checks to be skipped was set.
Merge pull request #21 from PagerDuty/environment-variable-to-skip-migration-check

use an environment variable to skip pending migrations check