chore: upgrade peer dependencies to include Angular 7

[r3hensler]

Please check if the PR fulfills these requirements

• [x] The commit message follows our guidelines: https://github.com/HackedByChinese/ng2-idle/blob/master/CONTRIBUTING.md#pr
• [ ] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (check one with "x")

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[x] Other... Please describe:

Allow Angular 7 core and common modules to be peer dependencies.

What is the current behavior? (You can also link to an open issue here)

When doing an npm ls --depth=0, the following errors are indicated when ng-idle core is being used with an Angular 7 application: npm ERR! peer dep missing: @angular/common@^6.0.0, required by @ng-idle/core@6.0.0-beta.3 npm ERR! peer dep missing: @angular/core@^6.0.0, required by @ng-idle/core@6.0.0-beta.3

What is the new behavior?

Angular 7 core and common modules can be peer dependencies.

Does this PR introduce a breaking change? (check one with "x")

[ ] Yes
[x] No

If this PR contains a breaking change, please describe the impact and migration path for existing applications: ...

Other information:

[moribvndvs]

A new build the version updated properly is available.