Set license to Apache-2.0 per rubygems-2.5 warning
Set license to Apache-2.0 per rubygems-2.5 warning

According to rubygems-2.5 the license should be listed at
http://spdx.org/licenses/

(I replaced a space with a dash, no actual change to licensing)

a-robinson/fluent-plugin-google-cloud,senthilkumar-kj/fluent-plugin-google-cloud,GoogleCloudPlatform/fluent-plugin-google-cloud,arthurdarcet/fluent-plugin-google-cloud,laramiel/fluent-plugin-google-cloud,cosmo0920/fluent-plugin-google-cloud