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

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