[ErrorReport] List the installed plugins

Adds the following section to the error report:

### Plugins

```
cocoapods            : 0.33.1
cocoapods-core       : 0.33.1
cocoapods-downloader : 0.6.1
cocoapods-plugins    : 0.2.0
cocoapods-trunk      : 0.1.3
cocoapods-try        : 0.3.0
```