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

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