bin/logstash-plugin update should ignore base gems

When we update a plugin in logstash it will do a check to make sure it ignore gem defined with a local path, with the move to the unified release the core gem are now always included in the repo.

We need to exclude the _core, *core-event-java, *core-plugin-api_ from the check to provide the user a less confusing message.

**Example**

``````
/t/logstash-5.0.0-alpha4-SNAPSHOT ❯❯❯ bin/logstash-plugin update logstash-filter-qatest ⏎
Update is not supported for manually defined plugins or local .gem plugin installations, skipping:
Updating logstash-filter-qatest
Updated logstash-filter-qatest 0.1.0 to 0.1.1
/t/logstash-5.0.0-alpha4-SNAPSHOT ❯❯❯ vim Gemfile
/t/logstash-5.0.0-alpha4-SNAPSHOT ❯❯❯ vim Gemfile.jruby-1.9.lock
/t/logstash-5.0.0-alpha4-SNAPSHOT ❯❯❯ bin/plugin
/t/logstash-5.0.0-alpha4-SNAPSHOT ❯❯❯ bin/logstash-plugin update ⏎
Update is not supported for manually defined plugins or local .gem plugin installations, skipping: logstash-core, logstash-core-event-java, logstash-core-plugin-api
Updating logstash-codec-collectd, logstash-codec-dots, logstash-codec-edn, logstash-codec-edn_lines, logstash-codec-es_bulk, logstash-codec-fluent, logstash-codec-graphite, logstash-codec-json, logstash-codec-json_lines, logstash-codec-line, logstash-codec-msgpack, logstash-codec-multiline, logstash-codec-netflow, logstash-codec-plain, logstash-codec-rubydebug, logstash-filter-clone, logstash-filter-csv, logstash-filter-date, logstash-filter-dns, logstash-filter-drop, logstash-filter-fingerprint, logstash-filter-geoip, logstash-filter-grok, logstash-filter-json, logstash-filter-kv, logstash-filter-metrics, logstash-filter-mutate, logstash-filter-qatest, logstash-filter-ruby, logstash-filter-sleep, logstash-filter-split, logstash-filter-syslog_pri, logstash-filter-throttle, logstash-filter-urldecode, logstash-filter-useragent, logstash-filter-uuid, logstash-filter-xml, logstash-input-beats, logstash-input-couchdb_changes, logstash-input-elasticsearch, logstash-input-exec, logstash-input-file, logstash-input-ganglia, logstash-input-gelf, logstash-input-generator, logstash-input-graphite, logstash-input-heartbeat, logstash-input-http, logstash-input-http_poller, logstash-input-imap, logstash-input-irc, logstash-input-jdbc, logstash-input-kafka, logstash-input-log4j, logstash-input-lumberjack, logstash-input-pipe, logstash-input-rabbitmq, logstash-input-redis, logstash-input-s3, logstash-input-snmptrap, logstash-input-sqs, logstash-input-stdin, logstash-input-syslog, logstash-input-tcp, logstash-input-twitter, logstash-input-udp, logstash-input-unix, logstash-input-xmpp, logstash-output-cloudwatch, logstash-output-csv, logstash-output-elasticsearch, logstash-output-file, logstash-output-graphite, logstash-output-http, logstash-output-irc, logstash-output-kafka, logstash-output-nagios, logstash-output-null, logstash-output-pagerduty, logstash-output-pipe, logstash-output-rabbitmq, logstash-output-redis, logstash-output-s3, logstash-output-sns, logstash-output-sqs, logstash-output-statsd, logstash-output-stdout, logstash-output-tcp, logstash-output-udp, logstash-output-webhdfs, logstash-output-xmpp, logstash-patterns-core
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