build: require clang >= 3.6

Also fix an unused lambda capture warning with clang >= 5.0

Change-Id: I29c56cdf560da4a44efa038d4e65d868975aab6e
diff --git a/tests/tools/nfdc/face-module.t.cpp b/tests/tools/nfdc/face-module.t.cpp
index 994c0b2..a3f4618 100644
--- a/tests/tools/nfdc/face-module.t.cpp
+++ b/tests/tools/nfdc/face-module.t.cpp
@@ -478,8 +478,7 @@
 
 BOOST_AUTO_TEST_CASE(MtuExistingFace)
 {
-  bool hasUpdateCommand = false;
-  this->processInterest = [this, &hasUpdateCommand] (const Interest& interest) {
+  this->processInterest = [this] (const Interest& interest) {
     this->respond409(interest, FacePersistency::FACE_PERSISTENCY_ON_DEMAND, 4000);
     // no command other than faces/create is expected
   };