fix(npm-merge-driver): only run if a `.git` folder is present

[glebec]

Assignee Tasks

• [ ] added unit tests (or none needed)
• [ ] written relevant docs (or none needed)
• [x] referenced any relevant issues (or none exist)

Closes #121 — maybe?

This needs actual testing with Heroku deployment. DO NOT MERGE 🛑 until properly tested.

[collin]

Success:

remote:        > boilermaker@1.0.0 prepare /tmp/build_3a6cd313b92cb2dc5fef4f1234d5b464
remote:        > if [ -d .git ]; then npm-merge-driver install; fi
remote:        
remote:        added 1115 packages from 1079 contributors and audited 9146 packages in 36.255s
remote:        found 0 vulnerabilities
remote:        
remote: 
remote: -----> Caching build
remote:        Clearing previous node cache
remote:        Saving 2 cacheDirectories (default):
remote:        - node_modules
    remote:        - bower_components (nothing to cache)
remote: 
remote: -----> Pruning devDependencies
remote:        
remote:        > husky@0.14.3 uninstall node_modules/husky
remote:        > node ./bin/uninstall.js
remote:        
remote:        husky
remote:        uninstalling Git hooks
remote:        done
remote:        
remote:        removed 925 packages and audited 531 packages in 10.844s
remote:        found 0 vulnerabilities
remote:        
remote: 
remote: -----> Build succeeded!