Fix parse error when .sqliterc option is '.timer ON'

[akicho8]

When .sqliterc option is .timer ON

migr8.rb:749:in `block in _get_migrations': undefined method `strip' for nil:NilClass (NoMethodError)
	from migr8/lib/migr8.rb:745:in `each_line'
	from migr8/lib/migr8.rb:745:in `_get_migrations'

Added .timer OFF to preamble.