suppress list source header if only one type requested with --quiet

##  Before
`gem list --quiet --no-versions`:
```bash

*** LOCAL GEMS ***

bundler
bundler-unload
fattr
rake
rubygems-bundler
rvm
session
tf
```
## After
`gem list --quiet --no-versions`:
```bash
bundler
bundler-unload
fattr
rake
rubygems-bundler
rvm
session
tf
```
I decided not to apply the change for `both?` as it would lead to confusion and does not seems as expected behavior.