refactor: remove deprecations, update to TypeScript 3.0+

[kamilmysliwiec]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

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

What is the current behavior?

Issue Number: #868

What is the new behavior?

Does this PR introduce a breaking change?

[x] Yes
[ ] No

Other information

[coveralls]

Pull Request Test Coverage Report for Build 1274

• 19 of 19 (100.0%) changed or added relevant lines in 9 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 93.807%

---

Totals
Change from base Build 1270:	0.04%
Covered Lines:	2824
Relevant Lines:	2945

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 1278

• 19 of 19 (100.0%) changed or added relevant lines in 9 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 93.807%

---

Totals
Change from base Build 1270:	0.04%
Covered Lines:	2824
Relevant Lines:	2945

---

💛 - Coveralls

[lock]

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