Improve member order of BoostVM

* The environment is the "main context" of the VM.
* Also follows better the constructor arguments order.

Author: eregon

vm/boostenv/main/boostvm-decl.hh

@@ -134,8 +134,8 @@ public:
   }
 
 public:
-  const VM vm;
   BoostEnvironment& env;
+  const VM vm;
   const VMIdentifier identifier;
 
 // Random number and UUID generation

vm/boostenv/main/boostvm.cc

@@ -39,8 +39,8 @@ BoostVM::BoostVM(BoostEnvironment& environment,
                  VMIdentifier identifier,
                  VirtualMachineOptions options,
                  std::unique_ptr<std::string>&& app, bool isURL) :
-  VirtualMachine(environment, options), vm(this),
-  env(environment), identifier(identifier),
+  VirtualMachine(environment, options), env(environment),
+  vm(this), identifier(identifier),
   uuidGenerator(random_generator),
   portClosed(false),
   _asyncIONodeCount(0),