Consider --force option when reinstalling package

[Comandeer]

Current version of updateLockFile function calls npm install without --force flag. Such method fails when package is a dev-dependency of itself (e.g. bundler, which bundles itself; it's the case with rollup and my project, in which I encountered this issue):

Travis CI log

	Error: Command failed: npm install -D --save-prefix="^" @comandeer/[email protected]
	npm ERR! code ENOSELF
	npm ERR! Refusing to install package with name "@comandeer/rollup-lib-bundler" under a package
	npm ERR! also called "@comandeer/rollup-lib-bundler". Did you name your project the same
	npm ERR! as the dependency you're installing?
	npm ERR! 
	npm ERR! For more information, see:
	npm ERR!     <https://docs.npmjs.com/cli/install#limitations-of-npms-install-algorithm>
	npm ERR! A complete log of this run can be found in:
	npm ERR!     /home/travis/.npm/_logs/2018-02-03T21_18_40_904Z-debug.log
	    at checkExecSyncError (child_process.js:601:13)
	    at execSync (child_process.js:641:13)
	    at updateLockfile (/home/travis/.nvm/versions/node/v8.9.4/lib/node_modules/greenkeeper-lockfile/lib/update-lockfile.js:44:7)
	    at Module.update [as exports] (/home/travis/.nvm/versions/node/v8.9.4/lib/node_modules/greenkeeper-lockfile/update.js:54:3)
	    at Object.<anonymous> (/home/travis/.nvm/versions/node/v8.9.4/lib/node_modules/greenkeeper-lockfile/update.js:62:37)
	    at Module._compile (module.js:643:30)
	    at Object.Module._extensions..js (module.js:654:10)
	    at Module.load (module.js:556:32)
	    at tryModuleLoad (module.js:499:12)
	    at Function.Module._load (module.js:491:3)

Addition of --force flag should be enough. It also should be safe in this context as greenkeeper just reinstalls already installed package.