Tests fail with latest master

### I'm Submitting a ...

```
[x] Bug report
[ ] Feature request
[ ] Support request
```

### Steps to Reproduce

1. `git clone git@github.com:arqex/react-datetime.git`
2. `cd react-datetime`
3. `npm install`
4. `npm run test:all`

### Expected Results

Tests pass

### Actual Results

Tests fail

```
Test Suites: 2 failed, 1 passed, 3 total
Tests: 26 failed, 2 skipped, 121 passed, 149 total
Snapshots: 24 failed, 1 passed, 25 total
```

### Minimal Reproduction of the Problem

### Other Information (e.g. stacktraces, related issues, suggestions how to fix)

This is with latest master https://github.com/arqex/react-datetime/commit/7e30d6c20cd864bf8e91bc94e6c3a0ee02864d19 using node version: v14.15.5

The same commit as ran on CI here https://travis-ci.org/github/arqex/react-datetime/builds/747440775
CI runs those tests with node 8 and latest stable (v15.3.0)

One thing that makes it hard to reproduce test results is there is no lock file committed to the project. So there is no way to tell if there are dependency issues causing failures.

I would like to propose:

- We commit the npm lock file (or yarn if the maintainers prefer)
- We add Dependabot (or another dependency manager) to the repo. Its free, and makes dependency updating straightforward https://github.blog/2020-06-01-keep-all-your-packages-up-to-date-with-dependabot/

I think if we make these changes then it will be easier to contribute back while ensuring code quality with reliable tests.