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

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