Allow versions longer than long.MaxValue

[pophils]

In a distributed environment where versions are generated using timestamp up to milliseconds level as shown below

unix_based_version_number=$(date '+%Y%m%d%H%M%S%s')

running migrate fails due to OverflowException. This is as result of the MigrationVersion supporting long values. Changing to decimal values seems to fix the issue as tested locally.