Fix rubocop violation for fluentd/fluent-plugin-loki (grafana#1646)

* Don't use sudo to install bundler

Follow [the official installation docs](https://github.com/rubygems/bundler#installation-and-usage).

* Fix rubocop viloation: kind_of? -> is_a?

```
lib/fluent/plugin/out_loki.rb:76:21: C: Style/ClassCheck: Prefer
Object#is_a? over Object#kind_of?.
        unless @uri.kind_of?(URI::HTTP) or @uri.kind_of?(URI::HTTPS)
                    ^^^^^^^^
```

* Fix rubocop violation

```
lib/fluent/plugin/out_loki.rb:78:9: C: Layout/EmptyLineAfterGuardClause:
Add empty line after guard clause.
        end
```

* Fix rubocup violation: or -> ||

```
lib/fluent/plugin/out_loki.rb:76:38: C: Style/AndOr: Use || instead of
or.
        unless @uri.is_a?(URI::HTTP) or @uri.is_a?(URI::HTTPS)
                                     ^^
```

* Fix rubocop violation: use single quoate

```
lib/fluent/plugin/out_loki.rb:77:38: C: Style/StringLiterals: Prefer
single-quoted strings when you don't need string interpolation or
special symbols.
          raise Fluent::ConfigError, "url parameter must be valid HTTP"
                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```

* Ignore rubocop violation: configure method is still not complex

Both rubocop and rspec fail with the current codes. Except for this
rubocop violation, they were fixed. this cyclomatic complexity violation
occurrs in configure method. However, it's still simple and the
implementation could be changed during spec fixation. So, this violation
should be ignored at this moment.

```
lib/fluent/plugin/out_loki.rb:72:7: C: Metrics/CyclomaticComplexity:
Cyclomatic complexity for configure is too high. [7/6]
      def configure(conf) ...
      ^^^^^^^^^^^^^^^^^^^
```

fluentd/fluent-plugin-grafana-loki/bin/setup

@@ -3,5 +3,5 @@ set -euo pipefail
 IFS=$'\n\t'
 set -vx
 
-sudo gem install bundle
+gem install bundler
 bundle install

fluentd/fluent-plugin-grafana-loki/lib/fluent/plugin/out_loki.rb

@@ -69,13 +69,14 @@ class LokiOutput < Fluent::Plugin::Output # rubocop:disable Metrics/ClassLength
         config_set_default :chunk_keys, []
       end
 
-      def configure(conf)
+      def configure(conf) # rubocop:disable Metrics/CyclomaticComplexity
         compat_parameters_convert(conf, :buffer)
         super
         @uri = URI.parse(@url + '/loki/api/v1/push')
-        unless @uri.kind_of?(URI::HTTP) or @uri.kind_of?(URI::HTTPS)
-          raise Fluent::ConfigError, "url parameter must be valid HTTP"
+        unless @uri.is_a?(URI::HTTP) || @uri.is_a?(URI::HTTPS)
+          raise Fluent::ConfigError, 'url parameter must be valid HTTP'
         end
+
         @record_accessors = {}
         conf.elements.select { |element| element.name == 'label' }.each do |element|
           element.each_pair do |k, v|