Upgrade rest-client to version 1.8.0

Upgrade rest-client to version 1.8.0

Author: yoshiekiura

Gemfile.lock

@@ -4,7 +4,7 @@ GIT
   specs:
     mina-slack (0.0.3)
       mina (~> 0.3.0)
-      rest-client (~> 1.6.7)
+      rest-client (>= 1.8.0)
 
 GIT
   remote: git://github.com/InfraexDev/unread.git
@@ -382,7 +382,7 @@ GEM
     responders (2.4.0)
       actionpack (>= 4.2.0, < 5.3)
       railties (>= 4.2.0, < 5.3)
-    rest-client (1.6.9)
+    rest-client (1.8.0)
       mime-types (~> 1.16)
     rotp (3.3.1)
     rspec-core (3.8.0)
@@ -551,7 +551,7 @@ DEPENDENCIES
   rake
   rbtree
   redis-rails
-  rest-client (~> 1.6.8)
+  rest-client (>= 1.8.0)
   rotp
   rspec-rails
   sass-rails (~> 5.0.7)