title: Can't update database using EF Migrations after updating to EF 6.0.0-alpha
body: <p>After updating my Entity Framework to version 6.0.0-alpha1 I cant update database.
  This is the error I'm getting:</p>
  
  <pre><code>PM&gt; update-database -verbose
  Using StartUp project 'DataCenter'.
  Using NuGet project 'DataCenter.Domain'.
  Specify the '-Verbose' flag to view the SQL statements being applied to the target database.
  Target database is: 'datacenter' (DataSource: ., Provider: System.Data.SqlClient, Origin: Configuration).
  Upgrading history table.
  ALTER TABLE [dbo].[__MigrationHistory] ADD [ContextKey] [nvarchar](512) NOT NULL DEFAULT 'DataCenter.Domain.Migrations.Configuration'
  ALTER TABLE [dbo].[__MigrationHistory] DROP CONSTRAINT [PK_dbo.__MigrationHistory]
  System.Data.SqlClient.SqlException (0x80131904): 'PK_dbo.__MigrationHistory' is not a constraint.
  Could not drop constraint. See previous errors.
     at System.Data.SqlClient.SqlConnection.OnError(SqlException exception, Boolean breakConnection, Action`1 wrapCloseInAction)
     at System.Data.SqlClient.SqlInternalConnection.OnError(SqlException exception, Boolean breakConnection, Action`1 wrapCloseInAction)
     at System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj, Boolean callerHasConnectionLock, Boolean asyncClose)
     at System.Data.SqlClient.TdsParser.TryRun(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject stateObj, Boolean&amp; dataReady)
     at System.Data.SqlClient.SqlCommand.RunExecuteNonQueryTds(String methodName, Boolean async, Int32 timeout)
     at System.Data.SqlClient.SqlCommand.InternalExecuteNonQuery(TaskCompletionSource`1 completion, String methodName, Boolean sendToPipe, Int32 timeout, Boolean asyncWrite)
     at System.Data.SqlClient.SqlCommand.ExecuteNonQuery()
     at System.Data.Entity.Migrations.DbMigrator.ExecuteSql(DbTransaction transaction, MigrationStatement migrationStatement)
     at System.Data.Entity.Migrations.Infrastructure.MigratorLoggingDecorator.ExecuteSql(DbTransaction transaction, MigrationStatement migrationStatement)
     at System.Data.Entity.Migrations.DbMigrator.ExecuteStatements(IEnumerable`1 migrationStatements)
     at System.Data.Entity.Migrations.Infrastructure.MigratorBase.ExecuteStatements(IEnumerable`1 migrationStatements)
     at System.Data.Entity.Migrations.DbMigrator.UpgradeHistory(IEnumerable`1 upgradeOperations)
     at System.Data.Entity.Migrations.Infrastructure.MigratorLoggingDecorator.UpgradeHistory(IEnumerable`1 upgradeOperations)
     at System.Data.Entity.Migrations.DbMigrator.Update(String targetMigration)
     at System.Data.Entity.Migrations.Infrastructure.MigratorBase.Update(String targetMigration)
     at System.Data.Entity.Migrations.Design.ToolingFacade.UpdateRunner.RunCore()
     at System.Data.Entity.Migrations.Design.ToolingFacade.BaseRunner.Run()
  ClientConnectionId:eda1a366-d3c8-44de-8cc9-8f64e2511b4e
  'PK_dbo.__MigrationHistory' is not a constraint.
  Could not drop constraint. See previous errors.
  </code></pre>
  
  <p>What's the cause of this? I've queried sysobjects table for 'PK_dbo.__MigrationHistory' but that does not exist.
  any idea?</p>
  
  <p>Thanks in advance.</p>