Add cgo build constraint for sqlite support (#25)

Required to upgrade https://github.com/mattn/go-sqlite3 which does not compile correctly with missing cgo.

pkg/dbmate/sqlite.go

@@ -1,3 +1,5 @@
+// +build cgo
+
 package dbmate
 
 import (
@@ -12,6 +14,11 @@ import (
 	_ "github.com/mattn/go-sqlite3" // sqlite driver for database/sql
 )
 
+func init() {
+	RegisterDriver(SQLiteDriver{}, "sqlite")
+	RegisterDriver(SQLiteDriver{}, "sqlite3")
+}
+
 // SQLiteDriver provides top level database functions
 type SQLiteDriver struct {
 }