Update dependency rollup to ^1.15.5

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
rollup	dependencies	patch	^1.15.4 -> ^1.15.5

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Release Notes

rollup/rollup

v1.15.5

Compare Source

2019-06-14

Bug Fixes

• Do not include any comments for completely tree-shaken files so that renderedLength === 0 is a reliable check (#​2940)
• Do not cause type errors when returning null from resolveId (#​2941)

Pull Requests

• #​2940: Completely omit files that do not have any included statements (@​lukastaegert)
• #​2941: Explicitly allow null as return value for various hooks (@​lukastaegert)

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