Update jake.rb

Currently the default engine was webkit out process. 
Webkit out process is going to be deprecated from 5.1.
Therfore we also need to change the default webkit engine to inprocess.
Merge pull request #438 from rhomobile/BLR_ENHANCEMENT_WM_DEFAULT_ENGINE

Enhancement SR: EMBPD00158375, from 5.1 the default webkit engine will be in process