Fail to compile on TravisCI

Tests of nodeos-reverse-proxy on TravisCI fails due to some problem when installing node-kexec itself:

> kexec@2.0.2 install /home/travis/build/piranna/nodeos-reverse-proxy/node_modules/kexec

> node-gyp rebuild

gyp WARN download NVM_NODEJS_ORG_MIRROR is deprecated and will be removed in node-gyp v4, please use NODEJS_ORG_MIRROR

gyp WARN download NVM_NODEJS_ORG_MIRROR is deprecated and will be removed in node-gyp v4, please use NODEJS_ORG_MIRROR

gyp WARN download NVM_NODEJS_ORG_MIRROR is deprecated and will be removed in node-gyp v4, please use NODEJS_ORG_MIRROR

make: Entering directory `/home/travis/build/piranna/nodeos-reverse-proxy/node_modules/kexec/build'

  CXX(target) Release/obj.target/kexec/src/kexec.o

In file included from ../src/kexec.cc:2:0:

/home/travis/.node-gyp/6.1.0/include/node/v8.h:341:1: error: expected unqualified-id before ‘using’

/home/travis/.node-gyp/6.1.0/include/node/v8.h:477:1: error: expected unqualified-id before ‘using’

/home/travis/.node-gyp/6.1.0/include/node/v8.h:871:1: error: expected unqualified-id before ‘using’

../src/kexec.cc:133:23: error: variable or field ‘init’ declared void

../src/kexec.cc:133:23: error: ‘Handle’ was not declared in this scope

../src/kexec.cc:133:36: error: expected primary-expression before ‘>’ token

../src/kexec.cc:133:38: error: ‘target’ was not declared in this scope

../src/kexec.cc:137:5: error: ‘init’ was not declared in this scope

../src/kexec.cc:42:13: warning: ‘void kexec(const v8::FunctionCallbackInfo<v8::Value>&)’ defined but not used [-Wunused-function]

make: *** [Release/obj.target/kexec/src/kexec.o] Error 1

make: Leaving directory `/home/travis/build/piranna/nodeos-reverse-proxy/node_modules/kexec/build'

gyp ERR! build error 

gyp ERR! stack Error: `make` failed with exit code: 2

gyp ERR! stack     at ChildProcess.onExit (/home/travis/.nvm/versions/node/v6.1.0/lib/node_modules/npm/node_modules/node-gyp/lib/build.js:276:23)

gyp ERR! stack     at emitTwo (events.js:106:13)

gyp ERR! stack     at ChildProcess.emit (events.js:191:7)

gyp ERR! stack     at Process.ChildProcess._handle.onexit (internal/child_process.js:204:12)

gyp ERR! System Linux 3.13.0-40-generic

gyp ERR! command "/home/travis/.nvm/versions/node/v6.1.0/bin/node" "/home/travis/.nvm/versions/node/v6.1.0/lib/node_modules/npm/node_modules/node-gyp/bin/node-gyp.js" "rebuild"

gyp ERR! cwd /home/travis/build/piranna/nodeos-reverse-proxy/node_modules/kexec

gyp ERR! node -v v6.1.0

gyp ERR! node-gyp -v v3.3.1

gyp ERR! not ok 

Maybe should we add some tests to be sure node-kexec works with latests versions of Node.js?

Reproduce the TravisCI installation failure with Node.js v6.1.0 and inspect the native addon entry point in src/kexec.cc alongside the reported node-gyp output. Verify that node-kexec installs successfully and that the existing tests pass in the affected TravisCI environment.