[JIRA] Fix get_plugins_info method request in need of trailing slash

[dephekt]

The Jira.get_plugins_info() method is currently broken. Calling this method results in a 404 client error:

requests.exceptions.HTTPError: 404 Client Error: Not Found for url: https://platformsh.atlassian.net/rest/plugins/1.0

The issue is the URL should have a trailing slash but we don't set trailing=True when calling the Jira.get() method in the return. This remedies that situation and fixes #703.

[gonchik]

@dephekt Thank you !