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

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