OC_User_IMAP supporting only TLSv1

[lsbbs]

Affected apps

user_external

Steps to reproduce

1. Use OC_User_IMAP for authentication
   {imapserver:143/imap/tls/readonly}
2. Disable TLSv1 on the IMAP server remaining TLSv1.1 and TLSv1.2

See closed issue for Nectcloud nextcloud/server#10328

[violoncelloCH]

related to #44
#44 is the reason for this issue
it's an issue with the used php dependency which seems to be deprecated