get/import/url: handle existing output directory (#2610)

* get/import/url: handle existing output directory

Currently if you run

```
dvc get https://github.com/iterative/example-get-started model.pkl --out .
```

it will nuke your cwd. To fix that we need to properly handle existing
directories, same as usual unix cli utilities do that.

* travis: specify python version when installing from chocolatey

Fixes #2609

scripts/ci/before_install.sh

@@ -48,7 +48,7 @@ if [[ "$TRAVIS_OS_NAME" == "windows" ]]; then
         choco install python2
         echo 'PATH="/c/Python27:/c/Python27/Scripts:$PATH"' >> env.sh
     else
-        choco install python
+        choco install python --version 3.7.4
         echo 'PATH="/c/Python37:/c/Python37/Scripts:$PATH"' >> env.sh
     fi
 elif [[ "$TRAVIS_OS_NAME" == "osx" ]]; then