Update dependency @opencensus/core to ^0.0.18

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This PR contains the following updates:

Package	Type	Update	Change
@opencensus/core	dependencies	patch	^0.0.17 -> ^0.0.18

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Release Notes

census-instrumentation/opencensus-node

v0.0.18

Compare Source

• Fix a TypeError of url.parse (#​640)
• Map http attributes to the stackdriver format
• Fix: silently ignore the invalid keys and values (#​664)
• Fix propagation breaking for https.get (#​657)
• Update dependency jaeger-client to ~3.16.0 (#​656)
• Fix the problem caused by multiple http2.connect calls (#​650)

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