gemspec: refactor definition

[daipom]

• Fix duplicated definition of simplecov.
• Move development dependencies to Gemfile.
• Stop forcing the use of the latest version of fluentd.
  • Since this prevents testing with old Ruby.
• Remove webrick, as it appears not to be used anymore.

Diff of Gemfile.lock

-GIT
-  remote: https://github.com/fluent/fluentd.git
-  revision: bbb87c033e8879d38a546cf099a50c62fa5f37e7
-  specs:
-    fluentd (1.19.0)
-      async-http (~> 0.86)
-      base64 (~> 0.2)
-      bundler
-      cool.io (>= 1.4.5, < 2.0.0)
-      csv (~> 3.2)
-      drb (~> 2.2)
-      http_parser.rb (>= 0.5.1, < 0.9.0)
-      io-event (< 1.11.0)
-      io-stream (< 0.8.0)
-      logger (~> 1.6)
-      msgpack (>= 1.3.1, < 2.0.0)
-      serverengine (>= 2.3.2, < 3.0.0)
-      sigdump (~> 0.2.5)
-      strptime (>= 0.2.4, < 1.0.0)
-      tzinfo (>= 1.0, < 3.0)
-      tzinfo-data (~> 1.0)
-      uri (~> 1.0)
-      webrick (~> 1.4)
-      yajl-ruby (~> 1.0)
-      zstd-ruby (~> 1.5)
-
 PATH
   remote: .
   specs:
@@ -67,6 +41,27 @@
     fiber-local (1.1.0)
       fiber-storage
     fiber-storage (1.0.1)
+    fluentd (1.19.0)
+      async-http (~> 0.86)
+      base64 (~> 0.2)
+      bundler
+      cool.io (>= 1.4.5, < 2.0.0)
+      csv (~> 3.2)
+      drb (~> 2.2)
+      http_parser.rb (>= 0.5.1, < 0.9.0)
+      io-event (< 1.11.0)
+      io-stream (< 0.8.0)
+      logger (~> 1.6)
+      msgpack (>= 1.3.1, < 2.0.0)
+      serverengine (>= 2.3.2, < 3.0.0)
+      sigdump (~> 0.2.5)
+      strptime (>= 0.2.4, < 1.0.0)
+      tzinfo (>= 1.0, < 3.0)
+      tzinfo-data (~> 1.0)
+      uri (~> 1.0)
+      webrick (~> 1.4)
+      yajl-ruby (~> 1.0)
+      zstd-ruby (~> 1.5)
     http_parser.rb (0.8.0)
     io-endpoint (0.15.2)
     io-event (1.10.2)
@@ -132,15 +127,14 @@

 DEPENDENCIES
   fluent-logger!
-  fluentd!
+  fluentd rake (>= 0.9.2) rspec (>= 3.0.0) rspec-its (>= 1.1.0)
-  simplecov
+  simplecov (>= 0.5.4) simplecov-vim test-unit timecop (>= 0.3.0)
-  webrick

 BUNDLED WITH
    2.6.8