Do not call `scoutapm_enable_instrumentation` unless monitoring is enabled

[asgrim]

heroku/heroku-buildpack-php#390 (comment)

In responding to this query, I noticed that as soon as \Scoutapm\Extension\PotentiallyAvailableExtensionCapabilities is created, the scoutapm_enable_instrumentation(true); call is made, regardless of whether monitoring is enabled.

If monitoring is disabled (e.g. SCOUT_MONITOR=false) then the ext should NOT be enabled.

[asgrim]

Fixed in #278