Agent ignores verify_server_cert, cannot verify self-signed certs

[axw]

Originally reported by @simitt:

Option verify_server_cert: false in config/elastic_apm.yml is ignored. Setting ELASTIC_APM_VERIFY_SERVER_CERT=false in ENV is also ignored. The ruby agent still tries to verify and fails with

[ElasticAPM] Couldn't establish connection to APM Server:
"#<OpenSSL::SSL::SSLError: SSL_connect returned=1 errno=0 state=error: certificate verify failed (self signed certificate in certificate chain)>"