Update gem dependencies to modern versions

- Update mini_portile2 from ~> 2.7 to ~> 2.8
- Update mocha from ~> 0.9.12 to ~> 2.7
- Update minitest from ~> 2.1.0 to ~> 5.25
- Update yard from ~> 0.9.11 to ~> 0.9

Author: JamieMagee

gpgme.gemspec

@@ -17,11 +17,11 @@ Made Easy). GnuPG Made Easy (GPGME) is a library designed to make access to
 GnuPG easier for applications. It provides a High-Level Crypto API for
 encryption, decryption, signing, signature verification and key management.}
 
-  s.add_runtime_dependency "mini_portile2", "~> 2.7"
+  s.add_runtime_dependency "mini_portile2", "~> 2.8"
 
-  s.add_development_dependency "mocha",     "~> 0.9.12"
-  s.add_development_dependency "minitest",  "~> 2.1.0"
-  s.add_development_dependency "yard",      "~> 0.9.11"
+  s.add_development_dependency "mocha",     "~> 2.7"
+  s.add_development_dependency "minitest",  "~> 5.25"
+  s.add_development_dependency "yard",      "~> 0.9"
   s.add_development_dependency "coveralls_reborn"
   s.add_development_dependency "byebug"
 end