Add install step to download gcc and gcc-c++ or npm install make will fail on CentOS 7

[warnite]

npm install kept failing for me on Centos 7. I had to install gcc and gcc-c++ to make the install work.

sudo yum install gcc gcc-c++

gyp ERR! build error gyp ERR! stack Error: make` failed with exit code: 2
gyp ERR! stack at ChildProcess.onExit (/usr/lib/node_modules/npm/node_modules.bundled/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:215:12)
gyp ERR! System Linux 3.10.0-327.36.3.el7.x86_64
gyp ERR! command "/usr/bin/node" "/usr/lib/node_modules/npm/node_modules/node-gyp/bin/node-gyp.js" "rebuild"
gyp ERR! cwd /home/thecenter/ark_server_manager/node_modules/ursa
gyp ERR! node -v v6.9.1
gyp ERR! node-gyp -v v3.4.0
gyp ERR! not ok