Rename Fortran requirement to fix option generation

So, `depends_on :fortran => :optional` will generate "--with-fortran"
rather than "--with-fortranRequired".

Library/Homebrew/dependency_collector.rb

@@ -93,7 +93,7 @@ class DependencyCollector
     when :macos      then MinimumMacOSRequirement.new(tags)
     when :mysql      then MysqlDependency.new(tags)
     when :postgresql then PostgresqlDependency.new(tags)
-    when :fortran    then FortranRequired.new(tags)
+    when :fortran    then FortranDependency.new(tags)
     when :tex        then TeXDependency.new(tags)
     when :clt        then CLTDependency.new(tags)
     when :arch       then ArchRequirement.new(tags)

Library/Homebrew/requirements/fortran_dependency.rb

 require 'requirement'
 
-class FortranRequired < Requirement
+class FortranDependency < Requirement
   fatal true
 
   default_formula 'gfortran'