Bump netlify-plugin-pushover version

[AshikNesin]

This version contains enhancement on how the 400 error messages are handled and notification sound for error message is changed to "siren"

Are you adding a plugin or updating one?

• Adding a plugin
• Updating a plugin

Plugin version diff
https://diff.intrinsic.com/netlify-plugin-pushover/0.0.4/0.0.5

Have you completed the following?

• Read and followed the plugin author guidelines.
• Included all required fields in your entry.
• Tested the plugin locally and on Netlify, using the plugin version stated in your entry.

Test plan
N/A

[ehmicky]

Thanks!