Fix existing package check during release

[fredimachado]

Fixes existing package checks and adds double-brackets for improved bash string comparison.

The _version variable doesn't actually exist in the script.

This will always return 404: https://api.nuget.org/v3/registration5-semver1/aspirant.hosting.0.0.4/0.0.4.json

This is the correct link: https://api.nuget.org/v3/registration5-semver1/aspirant.hosting/0.0.4.json

The double-brackets should fix the error on line 19.

This also fixes the Node.js warning:

[DamianEdwards]

Thank you!