It's not possible to override ENV var from config/cucumber.yml

If in `config/cucumber.yml` for any project env var is specified it is impossible to override it from CLI.

For example
```
# in config/cucumber.yml:
some_profile: SOME_VAR=1
```

```
$ parallel_cucumber --debug --cucumber-options ' SOME_VAR=2 -p some_profile' features/functional/some.feature
...
== Running command `cucumber ... SOME_VAR=2 ... SOME_VAR=1 features/functional/some.feature ...`
...
```

It seems there is a problem in [`batch_mapped_files`](https://github.com/badoo/parallel_cucumber/blob/9c992eb/lib/parallel_cucumber/helper/cucumber.rb#L20) function — it should handle duplicated ENV vars (and replace old ones cucumber config with new ones from CLI/--cucumber-options).

Open lib/parallel_cucumber/helper/cucumber.rb and inspect batch_mapped_files, then reproduce the issue with the parallel_cucumber command shown in the report. Done means an ENV value supplied through --cucumber-options takes precedence over the same variable from config/cucumber.yml without leaving conflicting assignments in the generated command.

Written by the indexing model from the issue text.