Update diff handler for npm project deps

[cdupuis]

Noticed two issues when testing the diff handler and update workflows:

• The current fingerprint value was wrong in the New Target Library message
• Updating the fingerprint to ^2.0.3 with npm install pug@^2.0.3 --save-exact would actually install 2.0.4 and the branch would diff from the fingerprint target. I changed this so that the package.json gets edited first and then run npm install. This correctly sets the new target in the package.json. I did the re-write using a regex replace to not mess up the entire file formatting.

fixes #105, fixes #106

[cdupuis]

Pull request auto merged by Atomist.

• 1 approved review by @slimslenderslacks
• 2 successful checks

[atomist:generated] [auto-merge:on-approve]