fix broken unit tests on Travis CI

1. It turns out that `device_id` is not defined in unit test environment
which makes the `assert(self.kosync_device_id)` fail and somehow makes `luajit` segmentation fault.
2. In the device spec, stubbing `io.open` will make subsquent `require`
all return boolean value and segmentation fault `luajit`.

This patch is a minimum viable change that makes CI work on both my
local box and Travis.

spec/unit/readerrolling_spec.lua

@@ -156,6 +156,9 @@ describe("Readerrolling module", function()
     end)
 
     describe("switching screen mode should not change current page number", function()
+        teardown(function()
+            readerui:handleEvent(Event:new("ChangeScreenMode", "portrait"))
+        end)
         it("for portrait-landscape-portrait switching", function()
             for i = 80, 100, 10 do
                 readerui:handleEvent(Event:new("ChangeScreenMode", "portrait"))