Update rubygem license metadata to Apache-2.0

Update the gemspec metadata to reflect the current LICENSE.

Previously the project was licensed under MIT but was changed in 8eb9dae to Apache-2.0

Signed-off-by: Ricky McMillen <ricky.mcmillen@gmail.com>

Author: richardmcmillen

sigdump.gemspec

@@ -10,7 +10,7 @@ Gem::Specification.new do |gem|
   gem.version     = Sigdump::VERSION
   gem.authors     = ["Sadayuki Furuhashi"]
   gem.email       = ["frsyuki@gmail.com"]
-  gem.license     = "MIT"
+  gem.license     = "Apache-2.0"
   gem.has_rdoc    = false
   gem.files       = `git ls-files`.split("\n")
   gem.test_files  = `git ls-files -- {test,spec,features}/*`.split("\n")