Use File.exist?, restoring TLS certificate verification

[nabramovitz]

Fixes #176.

File.exists? is not present on current Ruby, so the three guards in
Connection#open_ssl_socket raise NoMethodError before a socket is
opened.

The first of them is the only path that sets VERIFY_PEER — everything
else in open_ssl_socket runs with the ctx.verify_mode = VERIFY_NONE set
at the top — so on current Ruby, passing a trust store through
SSLParams(:ts_files) does not merely fail to verify, it raises, and the
only reachable option is an unverified TLS connection. The other two guards
affect cert_file and key_file the same way.

exist? has been the spelling since 1.9 and exists? was only ever an
alias, so this changes nothing on older Rubies.

Verified

Two RabbitMQ 4.2.9 brokers, both holding certificates from the same
throwaway CA — one issued to localhost, one to wrong.example.invalid:

before:  SSLParams(ts_files: ca) -> NoMethodError, both brokers

after:   SSLParams(ts_files: ca) -> connects to localhost
                                    refuses wrong.example.invalid
                                    (hostname does not match)
         ssl: true               -> accepts both, unchanged and as documented

Ruby 3.3.11, stomp 1.4.10.