Fix #50 to prevent infinite loop when redeploying same SHA

[RyanSept]

See #50

• We fix this to fetch the next version from Gigalixir instead of doing current version + 1 since doing the latter presumed every push will increment the version number on Gigalixir but that only happens if there are new changes to be rebuilt. By fetching the current version we ensure that we always exit if the expected new version is healthy.
• We add a MAX_RETRY_ATTEMPTS action input for better control over retry time