Like it is named in the other analogous methods.

And use it consistently.

These gem task checks for a specific string in the gem subcommand
output. However, if we are inside a bundled environment (`bundle exec
rake install` instead of `rake install`), the ruby process will require
`bundler/setup` first thing, and initialize a silent UI for rubygems. As
a result, the output string will be always empty and the condition for
success will always fail. So, even if installation succeeded, user will
always be notified of a failure like:

```
foo 1.0 built to pkg/foo-1.0.gem.
rake aborted!
Couldn't install gem, run `gem install /home/deivid/Code/bundler/tmp/1/bundled_app/pkg/foo-1.0.gem' for more detailed output
...
```

So, don't check for a specific string in the output and only rely on the
exit status.

This is leftover code that should've been removed on a previous PR.