Fix the repo url format

This fix is needed to allow the library to be installed using the Cocoapods podspec file. Without this change, you get this error while attempting to run `pod install`:

```
Installing react-native-background-upload (5.0.0)

[!] Error installing react-native-background-upload
[!] /usr/local/bin/git clone git+https://github.com/Vydia/react-native-background-upload.git /var/folders/rh/zjc3x5b91k3gsw1bsl91nw200000gn/T/d20190110-76297-1gnxeok --template= --single-branch --depth 1

Cloning into '/var/folders/rh/zjc3x5b91k3gsw1bsl91nw200000gn/T/d20190110-76297-1gnxeok'...
fatal: unable to find remote helper for 'git+https'
```

Author: matt-oakes

package.json

@@ -8,7 +8,7 @@
   },
   "repository": {
     "type": "git",
-    "url": "git+https://github.com/Vydia/react-native-background-upload.git"
+    "url": "https://github.com/Vydia/react-native-background-upload.git"
   },
   "keywords": [
     "NSURLSession",