Fixes test for Gemfile entry changes

Broken by fbe38c9e9d4fe9f82518e8ffc1d757459b0c5f1c
The problem with assert_gem is that we are testing there without space in front of `gem`

One way is to modify that regexp and do 

```ruby
/^gem\s+["']#{gem}["']$/
```

```ruby
/^[\s]gem\s+["']#{gem}["']$/
```