all: fix cgo compile failures on tip

Apparently the cgo typechecks get better on tip, so use C.int instead
of Go integers.

Build tip as part of the Travis build, so we can ensure that any
errors are resolved before they get released to a wider audience.

.travis.yml

@@ -19,6 +19,7 @@ go:
   - 1.9.x
   - 1.10.x
   - 1.11.x
+  - master
 
 before_install:
   - |

callback.go

@@ -368,7 +368,7 @@ func callbackRet(typ reflect.Type) (callbackRetConverter, error) {
 func callbackError(ctx *C.sqlite3_context, err error) {
 	cstr := C.CString(err.Error())
 	defer C.free(unsafe.Pointer(cstr))
-	C.sqlite3_result_error(ctx, cstr, -1)
+	C.sqlite3_result_error(ctx, cstr, C.int(-1))
 }
 
 // Test support code. Tests are not allowed to import "C", so we can't

sqlite3.go

@@ -1674,7 +1674,7 @@ func (c *SQLiteConn) prepare(ctx context.Context, query string) (driver.Stmt, er
 	defer C.free(unsafe.Pointer(pquery))
 	var s *C.sqlite3_stmt
 	var tail *C.char
-	rv := C._sqlite3_prepare_v2_internal(c.db, pquery, -1, &s, &tail)
+	rv := C._sqlite3_prepare_v2_internal(c.db, pquery, C.int(-1), &s, &tail)
 	if rv != C.SQLITE_OK {
 		return nil, c.lastError()
 	}
@@ -1718,7 +1718,7 @@ func (c *SQLiteConn) GetFilename(schemaName string) string {
 // GetLimit returns the current value of a run-time limit.
 // See: sqlite3_limit, http://www.sqlite.org/c3ref/limit.html
 func (c *SQLiteConn) GetLimit(id int) int {
-	return int(C._sqlite3_limit(c.db, C.int(id), -1))
+	return int(C._sqlite3_limit(c.db, C.int(id), C.int(-1)))
 }
 
 // SetLimit changes the value of a run-time limits.