Remove circleci support

[springmeyer]

We've been continuing to depend on travis for testing and building on both linux and osx. In #221 we looked at using circle for OS X (circle has the limitation of only being able to build for one platform per project), but never moved on that per #221 (comment).

However now I think we should disable circle support completely by:

• removing the circle.yml
• disabling the build at https://circleci.com/gh/mapbox/node-osrm/edit

The reason is that circle is currently failing due to xcode version issues:

# constructor: uses defaults with no parameter
dyld: lazy symbol binding failed: Symbol not found: _clock_gettime
  Referenced from: /Users/distiller/node-osrm/lib/binding/node-osrm.node
  Expected in: flat namespace

These are probably the same as Project-OSRM/osrm-backend#3297 and only manifest with specific xcode combinations which are non-critical to fix and blocked on a boost bug. So, while fixable I think this is such a low priority we should disable circle to reduce distracting build failures we don't have the focus to fix.

[danpat]

LGTM - given Project-OSRM/osrm-backend#3768, we should do the minimum here though.

[TheMarex]

Obsolete with 5.7