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.

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