Only show debugging info when using the verbose option

[pfrenssen]

Since #77 got in there is some unneeded chatter about the patch process added to the composer output:

Patches already gathered. Skipping
  - Removing symfony/class-loader (v2.8.14)
  - Installing symfony/class-loader (v2.8.15)
    Loading from cache

Patches already gathered. Skipping
  - Removing symfony/http-kernel (v2.8.14)
  - Installing symfony/http-kernel (v2.8.15)
    Loading from cache

Patches already gathered. Skipping
  - Removing symfony/routing (v2.8.14)
  - Installing symfony/routing (v2.8.15)
    Loading from cache

Patches already gathered. Skipping
  - Removing symfony/serializer (v2.8.14)
  - Installing symfony/serializer (v2.8.15)
    Loading from cache

Patches already gathered. Skipping
  - Removing symfony/validator (v2.8.14)
  - Installing symfony/validator (v2.8.15)
    Loading from cache

This information is only useful when debugging problems. Let's hide these messages unless composer is invoked with the -v option.