fix(deps): update dependency @google-cloud/common to ^0.26.0

[renovate]

This PR contains the following updates:

Package	Type	Update	Change	References
@​google-cloud/common	dependencies	minor	^0.25.1 -> ^0.26.0	source

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Release Notes

googleapis/nodejs-common

v0.26.0

Compare Source

Implementation Changes

• fix(typescript): Make ResponseCallback match subtype (#​271)
• fix: Do not retry streaming POST requests. (#​268)
• Don't publish sourcemaps (#​256)

Dependencies

• chore: Remove 'is' dependency (#​270)
• chore(deps): update dependency sinon to v7 (#​267)
• chore(deps): update dependency typescript to ~3.1.0 (#​259)

Internal / Testing Changes

• chores(build): run codecov on continuous builds (#​276)
• chore: update new issue template (#​274)
• chore: re-enable codecov (#​266)
• test: move install to system tests, and other tsconfig cleanup (#​269)
• Update kokoro config (#​264)
• docs: Remove appveyor badge from readme (#​262)
• Update CI config (#​258)
• build: prevent system/sample-test from leaking credentials
• Update the kokoro config (#​254)
• test: remove appveyor config (#​253)
• Update CI config (#​252)

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