Remove deprecation warning from CakePHP 3.6

[ludeus]

Please note that testMain() and testMainPlugin() from tests/TestCase/Shell/Task/MaintenanceModeTaskTest.php has been disabled.
The cake/bake plugin use Twig/Twig now, and I have lot of unresolved issues with that tests. I can't spend more time on it.

[codecov-io]

Codecov Report

Merging #15 into master will decrease coverage by 0.09%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##             master      #15     +/-   ##
===========================================
- Coverage     95.23%   95.14%   -0.1%     
  Complexity       44       44             
===========================================
  Files             7        7             
  Lines           105      103      -2     
===========================================
- Hits            100       98      -2     
  Misses            5        5

Impacted Files	Coverage Δ	Complexity Δ
src/Mode/View.php	93.75% <100%> (ø)	3 <0> (ø)	⬇️
src/Mode/Redirect.php	100% <100%> (ø)	3 <0> (ø)	⬇️
src/Middleware/MaintenanceMiddleware.php	89.47% <100%> (-0.27%)	22 <0> (ø)
src/Mode/Mode.php	100% <100%> (ø)	4 <0> (ø)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 3b9c4b6...5c53fca. Read the comment docs.

[ludeus]

Note that CakePHP 3.6 require PHP >= 5.6, no more 5.5. It is why the build failed.

[HavokInspiration]

I pushed a bunch of commits on top of your PR to take care of Bake, the failing builds and updating the README.

I'll release a new 3.0.0 version of the plugin today, when all builds pass.

Thanks a lot for your work updating the plugin !

[HavokInspiration]

@ludeus Version 3.0.0 just released.