Update rack requirement from ~> 1.4 to >= 1.4, < 4.0

Gemfile.lock

@@ -1,30 +1,37 @@
 PATH
   remote: .
   specs:
-    jimson (0.9.1)
-      blankslate (~> 3.1.2)
-      multi_json (~> 1.7.6)
-      rack (~> 1.4.5)
-      rest-client (~> 1.6.7)
+    jimson (0.10.0)
+      blankslate (>= 3.1.2)
+      multi_json (~> 1.0)
+      rack (>= 1.4, < 4.0)
+      rest-client (~> 1.0)
 
 GEM
-  remote: http://rubygems.org/
+  remote: https://rubygems.org/
   specs:
-    blankslate (3.1.2)
+    blankslate (3.1.3)
     diff-lcs (1.2.4)
+    domain_name (0.5.20190701)
+      unf (>= 0.0.5, < 1.0.0)
+    http-cookie (1.0.5)
+      domain_name (~> 0.5)
     json (1.8.0)
     json (1.8.0-java)
-    mime-types (1.23)
-    multi_json (1.7.6)
-    rack (1.4.5)
+    mime-types (2.99.3)
+    multi_json (1.15.0)
+    netrc (0.11.0)
+    rack (3.0.4.2)
     rack-test (0.6.2)
       rack (>= 1.0)
     rake (10.0.4)
     rcov (1.0.0)
     rdoc (4.0.1)
       json (~> 1.4)
-    rest-client (1.6.7)
-      mime-types (>= 1.16)
+    rest-client (1.8.0)
+      http-cookie (>= 1.0.2, < 2.0)
+      mime-types (>= 1.16, < 3.0)
+      netrc (~> 0.7)
     rspec (2.13.0)
       rspec-core (~> 2.13.0)
       rspec-expectations (~> 2.13.0)
@@ -33,6 +40,10 @@ GEM
     rspec-expectations (2.13.0)
       diff-lcs (>= 1.1.3, < 2.0)
     rspec-mocks (2.13.1)
+    unf (0.1.4)
+      unf_ext
+    unf (0.1.4-java)
+    unf_ext (0.0.8.2)
 
 PLATFORMS
   java

jimson.gemspec

@@ -12,7 +12,7 @@ spec = Gem::Specification.new do |s|
   s.add_dependency("blankslate", ">= 3.1.2")
   s.add_dependency("rest-client", "~> 1.0")
   s.add_dependency("multi_json", "~> 1.0")
-  s.add_dependency("rack", "~> 1.4")
+  s.add_dependency("rack", ">= 1.4", "< 4.0")
 
   s.files = %w[
     VERSION