Created .swift-version

This is to give let the pod spec validator know which Swift version to use otherwise we get the warning:

```shell
$ pod spec lint ReactiveReSwift.podspec

 -> ReactiveReSwift (4.0.0)
    - WARN  | swift_version: The validator for Swift projects uses Swift 3.0 by default, if you are using a different version of swift you can use a `.swift-version` file to set the version for your Pod. For example to use Swift 2.3, run: 
    `echo "2.3" > .swift-version`

Analyzed 1 podspec.

[!] The spec did not pass validation, due to 1 warning (but you can use `--allow-warnings` to ignore it).
```

.swift-version

@@ -0,0 +1 @@
+4.0