Fix Travis Jruby job error

[dks17]

I would like fix Travis Jruby job which was broken in #55

Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

Also I added two small changes:

Does actually bundler need in Gemfile?

gem 'bundler', '~> 1.1'

I think it is actually does't matter.
Value of what variable we should check retry_sleep or opts[:retry_sleep]?

retry_sleep = opts[:retry_sleep]
# code
if attempts_left > 0
  # if not passed a retry_sleep value, we sleep for the remaining life of the lock
  unless opts[:retry_sleep] # <==
    locked_until = Mongoid::Locker::Wrapper.locked_until(self)

[mongoid-bot]

	1 Warning
⚠️	Unless you’re refactoring existing code, please update CHANGELOG.md.

Here's an example of a CHANGELOG.md entry:

* [#58](https://github.com/mongoid/mongoid-locker/pull/58): Fix travis jruby job error - [@dks17](https://github.com/dks17).

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[dblock]

This technically doesn't matter, but the change is intentionally correct.

[dblock]

Merged. Do you want to add a specific JRuby version to the build matrix that's not allowed to fail? PR.

[dks17]

Merged. Do you want to add a specific JRuby version to the build matrix that's not allowed to fail? PR.

I just wanted fix one of test for jruby and investigated it. And I know that self.update wrapper method returns false when it should be true. The number of documents updated in the write is 0.

      def self.update(klass, query, setter)
        rc = klass.with(write: { w: 1 }) do
          klass.collection.find(query).update_one(setter)
        end
        rc.ok? && rc.documents.first['n'] == 1
      end

That behaviour might be the reason one of opened issues.