Update MySqlTypeMap.cs

version 1.1.2 causes error when migrating 'versioninfo' table by adding a 'description' column on MySQL database.
This error caused by method 'Up' in 'VersionDescriptionMigration' class. The method calls .AsString(int.MaxValue) which is beyond 
definition in MySqlTypeMap class
Merge branch 'patch-1' of https://github.com/eff0880/fluentmigrator into eff0880-patch-1
Merge pull request #461 of eff0880/patch-1

* eff0880-patch-1:
  Update MySqlTypeMap.cs