Update GNU Fortran documentation (6.1.0, 5.4.0, 4.9.3)

lib/docs/scrapers/gnu/gnu_fortran.rb

@@ -6,8 +6,14 @@ module Docs
       home: 'https://gcc.gnu.org/fortran/'
     }
 
+    version '6' do
+      self.release = '6.1.0'
+      self.dir = '/Users/Thibaut/DevDocs/Docs/gfortran6'
+      self.base_url = "https://gcc.gnu.org/onlinedocs/gcc-#{release}/gfortran/"
+    end
+
     version '5' do
-      self.release = '5.3.0'
+      self.release = '5.4.0'
       self.dir = '/Users/Thibaut/DevDocs/Docs/gfortran5'
       self.base_url = "https://gcc.gnu.org/onlinedocs/gcc-#{release}/gfortran/"
     end