enable npm caching in travis

[jhendrixMSFT]

Latest improvements:

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[AutorestCI]

Automation for azure-sdk-for-go

Unable to detect any generation context from this PR.

[AutorestCI]

Automation for azure-sdk-for-python

Unable to detect any generation context from this PR.

[AutorestCI]

Automation for azure-sdk-for-node

Unable to detect any generation context from this PR.

[AutorestCI]

Automation for azure-sdk-for-java

Unable to detect any generation context from this PR.

[AutorestCI]

Automation for azure-sdk-for-js

Unable to detect any generation context from this PR.

[AutorestCI]

Automation for azure-sdk-for-ruby

Unable to detect any generation context from this PR.

[jhendrixMSFT]

See https://docs.travis-ci.com/user/caching/#npm-cache

[azuresdkci]

Can one of the admins verify this patch?