Depend on offical azure-core

No need to depend on my fork anymore, they've just release fix
officially.

Gemfile

@@ -95,9 +95,6 @@ end
 group :storage do
   gem "aws-sdk", "~> 2", require: false
   gem "google-cloud-storage", "~> 1.3", require: false
-
-  # Contains fix to be able to test using StringIO
-  gem "azure-core", git: "https://github.com/dixpac/azure-ruby-asm-core.git"
   gem "azure-storage", require: false
 
   gem "mini_magick"

Gemfile.lock

@@ -6,15 +6,6 @@ GIT
     queue_classic (3.2.0.RC1)
       pg (>= 0.17, < 0.20)
 
-GIT
-  remote: https://github.com/dixpac/azure-ruby-asm-core.git
-  revision: 4403389747f44a94b73e7a7522d1ea11f8b1a266
-  specs:
-    azure-core (0.1.8)
-      faraday (~> 0.9)
-      faraday_middleware (~> 0.10)
-      nokogiri (~> 1.7)
-
 GIT
   remote: https://github.com/erikhuda/thor.git
   revision: df5ba2b653a28087b3617d6c082b00866b0c0d6c
@@ -134,6 +125,10 @@ GEM
     aws-sdk-resources (2.10.19)
       aws-sdk-core (= 2.10.19)
     aws-sigv4 (1.0.1)
+    azure-core (0.1.9)
+      faraday (~> 0.9)
+      faraday_middleware (~> 0.10)
+      nokogiri (~> 1.7)
     azure-storage (0.11.4.preview)
       azure-core (~> 0.1)
       faraday (~> 0.9)
@@ -488,7 +483,6 @@ DEPENDENCIES
   activerecord-jdbcsqlite3-adapter (>= 1.3.0)
   arel!
   aws-sdk (~> 2)
-  azure-core!
   azure-storage
   backburner
   bcrypt (~> 3.1.11)