Use correct suffix (.zip or .apk) when downloading app from a url.

md5calculator (used in lib/devices/android/android-common.js)
can only handle files that end in .ipa or .apk, not .zip.
Currently urls that end in .apk are downloaded as appium-app.zip
Merge pull request #1727 from rectalogic/master

Use correct suffix (.zip or .apk) when downloading app from a url.