Fix missing gitter links

[mayurkale22]

No description provided.

[mayurkale22]

@hekike please review. Looks like Gitter chat link is broken in npm: https://www.npmjs.com/package/@opencensus/nodejs-base

[codecov-io]

Codecov Report

Merging #519 into master will decrease coverage by 0.26%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #519      +/-   ##
==========================================
- Coverage   95.19%   94.93%   -0.27%     
==========================================
  Files         147      147              
  Lines       10562    10781     +219     
  Branches      919      912       -7     
==========================================
+ Hits        10055    10235     +180     
- Misses        507      546      +39

Impacted Files	Coverage Δ
test/test-http2.ts	77.83% <0%> (-18.99%)	⬇️
src/trace/propagation/noop-propagation.ts	40% <0%> (-10%)	⬇️
src/common/noop-logger.ts	11.11% <0%> (-5.56%)	⬇️
src/stackdriver-monitoring.ts	78.65% <0%> (-3.38%)	⬇️
test/test-ioredis.ts	97.16% <0%> (-0.55%)	⬇️
src/tracecontext-format.ts	96.42% <0%> (-0.19%)	⬇️
test/test-tracecontext-format.ts	99.06% <0%> (-0.04%)	⬇️
src/internal/string-utils.ts	100% <0%> (ø)	⬆️
test/test-tag-map.ts	100% <0%> (ø)	⬆️
test/test-metric-producer-manager.ts	100% <0%> (ø)	⬆️
... and 24 more

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[hekike]

lgtm, I've no review perm btw.