README steps for local development don't seem to work

[jquense]

Tell us about the issue

I was going to take a stab at adding support for array parameters in the filter plugins, but found that after cloning, setting up jruby, and bundler, it was impossible to install the gemspec dependencies. Presumably this is not an issue for core contributors using source versions of logstas, but I was not able to install the gems, due to incompatible dependency ranges on logstash-core:

Bundler could not find compatible versions for gem "logstash-core":
  In Gemfile:
    logstash-integration-jdbc was resolved to 5.2.4, which depends on
logstash-core-plugin-api (>= 1.60, <= 2.99) was resolved to 1.60.1, which
depends on
        logstash-core (>= 2.4.0.snapshot1, <= 2.4.99)

    logstash-integration-jdbc was resolved to 5.2.4, which depends on
      logstash-core (>= 6.5.0)

[andsel]

To develop a plugin you need a local clone of Logstash repository and then point the plugin's environment to it:
After created a clone of Logstash, go in the plugin directory and export:

export LOGSTASH_PATH=/path/to/logstash/ && export LOGSTASH_SOURCE=1

then

./gradlew vendor
buble install
bundle exec rspec