Make license SPDX compliant

Else `gem build` complains:

    WARNING:  WARNING: license value 'APLv2' is invalid.  Use a license identifier from
    http://spdx.org/licenses or 'Nonstandard' for a nonstandard license.

fluent-plugin-wendelin.gemspec

@@ -6,7 +6,7 @@ Gem::Specification.new do |gem|
     gem.summary         = %q{Fluentd output to Wendelin}
     gem.description     = %q{Fluentd output plugin to forward data to Wendelin system}
     gem.homepage        = "https://lab.nexedi.com/nexedi/fluent-plugin-wendelin"
-    gem.license         = "APLv2"
+    gem.license         = "Apache-2.0"
 
     gem.files           = `git ls-files -z`.split("\x0")
     gem.require_paths   = ["lib"]