Update dependency ts-jest to v26

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
ts-jest (source)	devDependencies	major	^25.0.0 -> ^26.0.0

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Release Notes

kulshekhar/ts-jest

### [`v26.0.0`](https://togithub.com/kulshekhar/ts-jest/blob/master/CHANGELOG.md#​2600-httpsgithubcomkulshekharts-jestcomparev2551v2600-2020-05-15) [Compare Source](https://togithub.com/kulshekhar/ts-jest/compare/v25.5.1...v26.0.0) ##### Bug Fixes - **compiler:** return `undefined` for `getScriptVersion` when a file doesn't exist in memory cache ([#​1641](https://togithub.com/kulshekhar/ts-jest/issues/1641)) ([6851b8e](https://togithub.com/kulshekhar/ts-jest/commit/6851b8e)) ##### Features - support Jest v26 ([#​1602](https://togithub.com/kulshekhar/ts-jest/issues/1602)) ([23b7741](https://togithub.com/kulshekhar/ts-jest/commit/23b7741)) ##### BREAKING CHANGES - Requires a minimum of TypeScript v3.8 - Drop support for Node 8 #### [25.5.1](https://togithub.com/kulshekhar/ts-jest/compare/v25.5.0...v25.5.1) (2020-05-09) ##### Bug Fixes - **compiler:** don't resolve files from build folder for `projectReferences` ([#​1614](https://togithub.com/kulshekhar/ts-jest/issues/1614)) ([74b92d3](https://togithub.com/kulshekhar/ts-jest/commit/74b92d3)) - **config:** don't set `include` value of `tsconfig` to empty array ([#​1606](https://togithub.com/kulshekhar/ts-jest/issues/1606)) ([8a29aaa](https://togithub.com/kulshekhar/ts-jest/commit/8a29aaa))

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