feat(core): upgrade to angular 6

[fulls1z3]

** PR Checklist Please check if your PR fulfills the following requirements:

• [x] The commit message follows our guidelines: https://github.com/fulls1z3/ngx-meta/blob/master/CONTRIBUTING.md#commit
• [x] Tests for the changes have been added (for bug fixes / features)
• [x] Docs have been added / updated (for bug fixes / features)

** PR Type What kind of change does this PR introduce?

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

** What is the current behavior?

Issue Number: N/A

** What is the new behavior?

** Does this PR introduce a breaking change?

[ ] Yes
[x] No

** Other information