added android sdk installation instructions to travis config.
added android sdk installation instructions to travis config.

frolovs/selendroid,christiannolte/selendroid,mach6/selendroid,PrakashGoulla/selendroid,selendroid/selendroid,ghrud92/selendroid,mach6/selendroid,smiklosovic/selendroid,sri096/selendroid,selendroid/selendroid,ghrud92/selendroid,luohaoyu/selendroid,vishals79/appium-uiautomator2-driver,luohaoyu/selendroid,ghrud92/selendroid,masbog/selendroid,appium/selendroid,anandsadu/selendroid,0x1mason/selendroid,smiklosovic/selendroid,koichirok/selendroid,asm89/selendroid,SpencerMalone/selendroid,lukeis/selendroid,anandsadu/selendroid,sri096/selendroid,smiklosovic/selendroid,luohaoyu/selendroid,anandsadu/selendroid,0x1mason/selendroid,rasata/selendroid,uchida/selendroid,lukeis/selendroid,mach6/selendroid,christiannolte/selendroid,frolovs/selendroid,appium/selendroid,asm89/selendroid,mach6/selendroid,PrakashGoulla/selendroid,sri096/selendroid,DominikDary/selendroid,koichirok/selendroid,christiannolte/selendroid,asm89/selendroid,smiklosovic/selendroid,masbog/selendroid,0x1mason/selendroid,koichirok/selendroid,rasata/selendroid,asm89/selendroid,DominikDary/selendroid,mach6/selendroid,christiannolte/selendroid,koichirok/selendroid,sri096/selendroid,VikingDen/selendroid,appium/selendroid,DominikDary/selendroid,SpencerMalone/selendroid,luohaoyu/selendroid,koichirok/selendroid,SpencerMalone/selendroid,vishals79/appium-uiautomator2-driver,masbog/selendroid,smiklosovic/selendroid,0x1mason/selendroid,SpencerMalone/selendroid,VikingDen/selendroid,uchida/selendroid,ghrud92/selendroid,luohaoyu/selendroid,lukeis/selendroid,lukeis/selendroid,selendroid/selendroid,masbog/selendroid,christiannolte/selendroid,vishals79/appium-uiautomator2-driver,anandsadu/selendroid,PrakashGoulla/selendroid,frolovs/selendroid,rasata/selendroid,uchida/selendroid,sri096/selendroid,frolovs/selendroid,uchida/selendroid,PrakashGoulla/selendroid,VikingDen/selendroid,vishals79/appium-uiautomator2-driver,anandsadu/selendroid,SpencerMalone/selendroid,selendroid/selendroid,selendroid/selendroid,rasata/selendroid,masbog/selendroid,ghrud92/selendroid,asm89/selendroid,uchida/selendroid,lukeis/selendroid,frolovs/selendroid,appium/selendroid,vishals79/appium-uiautomator2-driver,VikingDen/selendroid,PrakashGoulla/selendroid,appium/selendroid,rasata/selendroid