updated wx plugin to be up-to-date with the latest wx and Pillow

[yueranyuan]

updates function names to get rid of 2 deprecation warnings:
pyscreenshot/plugins/wxscreen.py:24: wxPyDeprecationWarning: Call to deprecated item EmptyBitmap. Use :class:'wx.Bitmap' instead
pyscreenshot/plugins/wxscreen.py:28: wxPyDeprecationWarning: Call to deprecated item ImageFromBitmap. Use bitmap.ConvertToImage instead. myWxImage = wx.ImageFromBitmap(bmp)

wrap input to 1 function to fulfill the latest function argument of frombytes in Pillow==5.2.0 to prevent error:
TypeError: argument 1 must be string or read-only buffer, not bytearray

[coveralls]

Coverage decreased (-0.1%) to 39.44% when pulling 398ee0a on yueranyuan:update_wx_plugin into 2730487 on ponty:master.

[ponty]

Travis CI fails: "TypeError: String or Unicode type required"
Ubuntu 18.04 also fails.
Bitmap call is not compatible with wxPython Classic.
Please check it!

[yueranyuan]

I think the constructor was changed in wxPython >= 4.0 to allow for wxBitmap(width, height). Defaulting to prior behaviour for wxPython < 4.0

[ponty]

Thanks!