mathn has been gemified in ruby 2.5

Gemfile

@@ -154,3 +154,7 @@ end
 gem "ibm_db" if ENV["IBM_DB"]
 gem "tzinfo-data", platforms: [:mingw, :mswin, :x64_mingw, :jruby]
 gem "wdm", ">= 0.1.0", platforms: [:mingw, :mswin, :x64_mingw, :mswin64]
+
+platforms :ruby_25 do
+  gem "mathn"
+end

Gemfile.lock

@@ -209,6 +209,7 @@ GEM
       nokogiri (>= 1.5.9)
     mail (2.6.5)
       mime-types (>= 1.16, < 4)
+    mathn (0.1.0)
     metaclass (0.0.4)
     method_source (0.8.2)
     mime-types (3.1)
@@ -396,6 +397,7 @@ DEPENDENCIES
   kindlerb (~> 1.2.0)
   libxml-ruby
   listen (>= 3.0.5, < 3.2)
+  mathn
   minitest (< 5.3.4)
   mocha (~> 0.14)
   mysql2 (>= 0.4.4)